GB2400222A

GB2400222A - Railway train detection system

Info

Publication number: GB2400222A
Application number: GB0307509A
Authority: GB
Inventors: Trevor Edwin Clegg
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-04-01
Filing date: 2003-04-01
Publication date: 2004-10-06
Anticipated expiration: 2023-04-01
Also published as: GB0307509D0; GB2400222B

Abstract

A train detection system includes a track circuit, a transmitter 5 and a receiver 8. A pulse modulated signal is transmitted across the circuit. The presence of a train between the transmitter and receiver is evidenced by attenuation of the signal due to the shunting effect of train axles. Faults in the rail track within the circuit may also be detected.

Description

N' 2400222 Railway Train Detection SYstem

Description

This invention relates to a railway train detection system.

Railway track circuits are extensively used as a means of train detection on a railway system. They generally comprise a transmitter for transmitting a signal to one end of a section of track that is suitably terminated at its ends to form a track circuit and a receiver for receiving the signal at the other end of the track circuit.

The general operation of the track circuit is that, in the absence of a train on the track circuit, the track circuit signal at the receiver is of sufficient amplitude that the receiver detects that the track circuit is clear of any significant attenuation and, under these conditions, the receiver provides an output signal that is commonly used to energise a track relay that indicates that the track section is unoccupied.

When a train enters into the track circuit, the train axles form a low impedance path across the track circuit. This low impedance path shunts the track resulting in a reduction or loss of receiver input signal. Under this condition, the receiver output signal is de-energised.

Equipment and system failures that stop the track circuit from performing its function should result in 'fail safe' condition. This 'fail safe' condition should result in the receiver output de-energising. This is commonly termed a 'right side failure'.

It is common for signals used in track circuits to contain some form of modulation so that it is unlikely that, under track or system fault conditions, a receiver can interpret the signal from an adjacent track circuit or noise source as a valid signal. This modulation is commonly frequency shift keyed (FSK). The signal level across the track is commonly within the range 1V to 6V under track unoccupied conditions, dependent on such factors as track circuit length, ballast leakage conductance and track circuit topography.

Equipment and system failures that incorrectly identify a track circuit as being unoccupied when the track circuit is occupied are potentially very dangerous and are commonly termed a 'wrong side failure'. This would occur if the receiver output were to be energised under a track circuit occupied condition.

Wrong side failures have been experienced on railway systems as a result of the axles of a train not reliably making a good contact with the rails as a result of rail surface contamination. This rail contamination is encountered under conditions such as leaf fall in autumn, where the rails are covered in an electrically insulating film, and also under conditions of tracks that see infrequent traffic, where the rail surface has a tendency to have a build up of surface oxidization.

Track circuits are in use where the transmitter signal is a voltage pulse which results in a relatively high voltage pulse in the order of 20V to 100V being applied across the track under track unoccupied conditions at regular intervals. This elevated voltage assists in breaking down the rail surface contamination when a train is present on the track, thereby improving the shunting effect of train axles under rail contamination conditions.

This invention is for a train detection system that has the combined advantages of both high voltage pulse transmitter signals coupled with pulse modulation techniques that ensures that the code used in the track circuit differs from the code used in other track circuits in the vicinity.

An object of this invention is to provide a means of detecting the presence or absence of a train on a section of railway track.

Another object of this invention is to minimise the possibility that rail surface contamination results in an inability to detect the presence of a train on a section of railway track.

Another object of this invention is to detect any discontinuities in the section of track as occurs in the event of rail fracture.

Another object of this invention is to ensure that faults on the railway system do not result in a condition where an occupied section of track is shown as unoccupied.

The above objects can be obtained by a train detection system comprising a transmitter and at least one receiver connected via a section of railway track used to ascertain the presence or absence of a train by establishing the attenuation of the transmitted signal as a result of the shunting effect of train axles where the transmitted signal applied to the track is in the form of pulses that are coded using pulse modulation techniques. In this invention the coded signal is transmitted as a series of pulses7 or bursts of audio frequency electrical signal to represent each pulse. The pulse coding can be controlled by either pulse position modulation (PPM)7 pulse width modulation (PWM) or a combination of both modulation techniques so as to encode the signal with a code dedicated to the track circuit.

Other track circuits in the vicinity would be encoded with differing codes.

The receipt of correctly coded signal with sufficient amplitude at the receiver indicates that the track circuit is un-shunted and also that there are no discontinuities within the track circuit. The output of the receiver under these conditions may be used to energise a track circuit relay7 the contacts of which are used to identify the occupation state of the track circuit. Alternatively the receiver may provide an output signal that identifies the occupation state of the track circuit. This signal may be connected directly to appropriate railway signalling and monitoring equipment.

In the event that signals are fed to the receiver from other sources or track circuits7 these signals will not have the modulation characteristics required by the receiver energise its output.

Existing coded track circuits use signals that have peak levels in the range 1V to 6V. The use of pulses7 or bursts of audio frequency signal to represent each pulse7 enables the signal amplitude during the pulse duration to be significantly higher for a given transmitter power than would be the case if the signal were to be applied continuously.

This enhanced voltage improves the capability of the signal in the track circuit to breakdown the insulating properties of any rail surface contamination that can exist between the rail and a train wheel. This rail contamination is encountered under conditions such as leaf fall in autumn7 where the rails are covered in an electrically insulating film7 and also under conditions of tracks that see infrequent traffic7 where the rail surface has a tendency to have a build up of surface oxidization.

This enhanced voltage also provides significantly improved signal to noise ratio margins at the receiver. This is of particular relevance to electrified railways where traction current interference levels can be significant in comparison with track circuit signal levels.

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Another feature that can be incorporated is the inclusion of additional variable information that can be incorporated into the pulse train that permits information to be passed by the transmitter to trains within the track circuit by means of electromagnetic coupling from track to the train via a suitable antenna mounted on the train.

The PPM and PWM codes may be implemented by the use of unidirectional polarity electrical pulses, pulses of varying polarity, representation of the pulses by electrical alternating signal bursts at frequencies within the frequency range 200Hz to 20kHz with either single or multiple frequencies representing the pulses. Alternatively, the pulse may be represented by the change in frequency of a carrier signal to produce a frequency shift keyed (FSK) signal or the change in phase of a carrier signal to produce a phase shift keyed (PSK) signal or a differential phase shift keyed (DPSK) signal.

The pulse position within the PPM may represent a binary value, a multi state value (commonly known as 'm-ary' where 'm' is an integral number of states) or represent an analogue value directly. In each case the interval between successive pulses determines the value.

The pulse width within the PWM may represent a binary value, a multi state value (commonly known as 'm-ary' where 'm' is an integral number of states) or represent an analogue value directly. In each case the width of the pulse determines the value.

The track circuit termination can use any standard existing track circuit terminating means. These may be by the introduction of insulated rail joints (IRJs) where the track circuit is terminated by rail electrical discontinuity, by the use of terminating bonds connected across the track to electrically separate adjacent contiguous track circuits in conjunction with simple tuned areas formed by capacitor tuning the inductance of sections of track abutting the terminating bonds or by the use of a compound tuned area system situated between two contiguous track circuits.

The receiver may comprise two separate channels to provide for checking the integrity of the received signal. These separate channels may use diverse means of establishing the content of the received signal and these channels may be arranged to alternately energise in sequence to provide indication of the presence of a valid track clear signal.

The following is a description, by way of example only, of a possible method of carrying the invention into effect with reference to the accompanying drawing in which: Figure 1 shows a block diagram of the track circuit Figure 2 shows a block diagram of the operational sections within the transmitter Figure 3 shows the schematic diagram of a track tuning unit Figure 4 shows a block diagram of the operational sections of the signal recovery and decode sections of the receiver Figure 5 shows a simplified schematic diagram of the safe enable track circuit relay drive section of the receiver.

Figure 6 shows a possible valid pulse position modulation code sequence cycle.

Figure 7 shows the timing sequence and representative waveforms at salient points in the system.

A section of railway track configured as a jointless track circuit is shown in Figure 1 where 1 and 2 are the running rails of the track.

A transmit end terminating bond 3 connects the rails 1 and 2 across the track at the transmit end of the track circuit. A receive end terminating bond 7 connects the rails 1 and 2 across the track at the receive end of the track circuit.

The transmitter 5 feeds a coded signal to the transmit end tuning unit 4 which is arranged to resonate with the inductance of the track section between terminating bond 3 and the transmit end tuning unit 4 at a nominal carrier frequency. This results in the transmitter signal being applied to the section of track between 4 and 6.

The receive end tuning unit 6 is arranged to resonate with the inductance of the track section between the receive end tuning unit 6 and receive end terminating bond 7. The signal from the receive end tuning unit 6 is fed to the receiver 8. The receiver 8 examines the received signal for both amplitude and code content and, in the event of the signal containing the valid code for the track circlet and of sufficient amplitude, energises the track circuit relay 9. Contacts on the track circuit relay 9 are used to pass information as to the occupancy of the track circuit to the signalling and control functions of the railway.

The system is arranged such that the track circuit relay 9 is energised only if the signal presented to the receiver 8 from the track circuit contains valid track circuit signal code at a level above a set threshold. This occurs only if the track circuit components are operating correctly and are in an intact state with no shunt applied across the track within the track circuit. When a train axle connects the rails within the track circuit, the received signal is attenuated and the track circuit relay is de-energised.

The block diagram of the transmitter is shown in Figure 2.

A pulse train generator 10 produces a coded pulse sequence that is allocated to the track circuit. This pulse sequence is fed to a modulator 12 together with the signal from a carrier frequency generator 11. The output of the modulator 12 is the carrier frequency gated by the coded pulse sequence resulting in carrier frequency bursts corresponding to the pulse sequence. This signal is passed via a bandpass filter 13 to a power amplifier 14 for onward transmission to the track circuit via the transmit end tuning unit 4.

The schematic diagram of a tuning unit is shown in Figure 3. This tuning unit schematic diagram is common to both transmit end tuning unit 4 and receive end tuning unit 6.

When applied as the transmit end tuning unit 4, the output signal of the transmitter 5 is fed into winding 1 5a of the transformer 15. The winding 15b is coupled to the track via a resonating capacitor 16 which forms a resonant circuit with the inductance of the section of track between the transmit end tuning unit 4 and the transmit end terminating bond 3.

When applied as the receive end tuning unit 6, the track signal is coupled to winding 1 5b via the resonating capacitor 16 which forms a resonant circuit with the inductance of the section of track between the receive end tuning unit 6 and the receive end terminating bond 7. The winding 1 5a is connected to the input of the receiver.

The block diagram of the signal recovery and decode sections of the receiver 8 is shown in Figure 4. This is divided into two separate sections illustrated as 1 7a to 20a inclusive and 1 7b to 20b inclusive. These circuits perform a similar function as each other but differ from each other in the use of diverse circuitry to minimise the probability of undetected failure modes as a result of common mode failures. Each section is arranged to identify a particular received signal code sequence above a preset threshold. The signal code sequence that is identified by each section differs from the code sequence identified by its partner.

The signal from the receive end tuning unit 6 is fed to the two receiver decode sections. In the upper section, the input signal is fed via a bandpass filter 1 7a to a combined demodulator and threshold detector 18a which extracts the pulse information from the received signal. This is passed to the pulse decoder 1 9a which, in the event of its seeing a particular code sequence provides an output pulse of preset duration to the input of the optocoupler 20a.

Similarly, in the lower section, the input signal is fed via a bandpass filter 1 7b to a combined demodulator and threshold detector 1 8b that extracts the pulse information from the received signal. This is passed to the pulse decoder 1 9b which, in the event of its seeing a particular code sequence provides an output pulse of preset duration to the input of the optocoupler 20b.

The functions performed by the decode sections may be implemented as conventional hardware or may employ microprocessor, microcontroller or digital signal processor (DSP) techniques to perform some or all of the functions.

The safe enable track circuit relay drive section of the receiver is shown in Figure 5.

This section is fed with a do supply between positive supply rail 21 and negative supply rail 22.

In the event of the optocouplers 20a and 20b being energised sequentially as a result of alternate receiver decode section channel valid code sequence detection for durations less than the intervening period between successive differing pulse sequences, capacitor C1 will be successively charged with opposite polarity via Da/Ra and Db/Rb. The sequential switching action results in C2 being charged to a negative voltage with respect to line 22 as a result of diode pump action through D1 and D2.

This negative voltage can only exist if both optocoupler channels are correctly and sequentially switched.

The presence of the negative signal across C2 results in a square wave generator 23 being energised. When it is energized, the output signal of the square wave generator is passed to the primary of transformer 24.

The split secondary of transformer 24 is connected to a full wave rectifier section comprising D3 and D4, smoothed and deglitched by C3 to provide an output for feeding the track circuit relay 9.

The track circuit relay 9 is therefore energised only when successive valid pulse sequences, at levels above a set threshold, are received sequentially from each of the signal decode sections of the receiver.

For illustration of the operation of the system in the following sections, the pulse sequences are shown with pulse position modulation (PPM) encoding. Pulse width modulation (PWM) encoding or a combination of PWM and PWM could be used equally well.

In order to illustrate the operation of the system, a valid pulse position modulation code sequence cycle will be defined as shown in Figure 6.

This illustrates a pulse train in which the intervals between pulses indicate the pulse position modulation value. In this instance it is shown as an asynchronous ternary (3 state) PPM code where successive pulse intervals represent, with increasing duration, the values a, b or c. In this illustration, the corresponding pulse position modulation intervals for 'a', 'b' and 'c' are 20ms, Sums and 40ms respectively. For this illustration, the code sequences 'acb' and 'ace' are included sequentially as the cycle sequence. In each case the value 'a' is used to indicate the start of the PPM code sequence. The variable duration shown as 'x' is included to maintain a constant period between the start of each successive PPM code sequences. In this illustration the start of each of the PPM code sequences is spaced at 1 Sums intervals and the cycle sequence duration is 260ms.

The timing sequence and representative waveforms at salient points in the system for the repetitive cycle sequence are shown in Figure 7.

The PPM signal at the output of the pulse train generator 10 section of the transmitter is shown in Figure 7A.

The envelope of the output of the modulator 12 section of the transmitter is shown in Figure 7B.

At the receiver the received signal envelope at the input to the demodulators 18a and 18b will be of the form shown in Figure 7C as a result of filter and track tuned area response to the signal waveform.

The output of each of the demodulator sections 1 8a and 1 8b of the receiver following threshold detection is shown in Figure 7D.

The PPM decoders 1 9a and 1 9b respond when they each detect their respective code sequences. The output of 1 9a pulses high on receipt of a valid 'acb' code as shown in Figure 7E. The output of 1 9b pulses high on receipt of a valid 'ace' code as shown in Figure 7F. These alternately energise the optocouplers 20a and 20b respectively.

The resultant voltage across capacitor C1 is shovvn in Figure 7G. This is sequentially charged via Db and Rb when optocoupler 20a is energised and via Da and Ra when optocoupler 20b is energised.

The switching sequence of optocouplers 20a and 20b transfers charge via diode pump action of D1 and D2 onto capacitor C2 which establishes a negative supply line with respect to negative supply rail 22 as shown in Figure 7H.

The presence of the negative signal across C2 results in a square vvave generator 23 being energised. When it is energised, the output signal of the square wave generator is passed to the primary of transformer 24.

Claims

Claims 1. A railway train detection track circuit system comprising a

transmitter and at least one receiver connected via a section of railway track used to ascertain the presence or absence of a train by establishing the attenuation of the transmitted signal as a result of the shunting effect of train axles where the transmitted signal applied to the track is in the form of pulses that are coded using pulse modulation techniques.
2. A railway train detection track circuit system comprising a transmitter and at least one receiver connected via a section of railway track used to ascertain the presence or absence of a train by establishing the attenuation of the transmitted signal as a result of the shunting effect of train axles where the transmitted signal applied to the track is in the form of pulses modulated with pulse position modulation.
3. A railway train detection track circuit system comprising a transmitter and at least one receiver connected via a section of railway track used to ascertain the presence or absence of a train by establishing the attenuation of the transmitted signal as a result of the shunting effect of train axles where the transmitted signal applied to the track is in the form of pulses modulated with pulse width modulation.
4. A railway train detection track circuit system comprising a transmitter and at least one receiver connected via a section of railway track used to ascertain the presence or absence of a train by establishing the attenuation of the transmitted signal as a result of the shunting effect of train axles where the transmitted signal applied to the track is in the form of pulses modulated with a combination of pulse width modulation and pulse position modulation.
5. A railway train detection system as defined in any of the claims 1 to 4 inclusive where the pulses are of unidirectional polarity.
6. A railway train detection system as defined in any of the claims 1 to 4 inclusive where the pulses are of varying polarity.
7. A railway train detection system as defined in any of the claims 1 to 4 inclusive where the pulses are represented by alternating signal bursts at a single frequency within the frequency range 200Hz to 20kHz.
8. A railway train detection system as defined in any of the claims 1 to 4 inclusive where the pulses are represented by alternating signal bursts at more than one frequency within the frequency range 200Hz to 20kHz.
9. A railway train detection system as defined in any of the claims 1 to 4 inclusive where the pulses are represented by a change in frequency of a carrier signal to produce a frequency shift keyed (FSK) signal.
10. A railway train detection system as defined in any of the claims 1 to 4 inclusive where the pulses are represented by a change in phase of a carrier signal to produce a phase shift keyed (PSK) signal.
11. A railway train detection system as defined in any of the claims 1 to 4 inclusive where the pulses are represented by a change in phase of a carrier signal to produce a differential phase shift keyed (DPSK) signal.
12. A railway train detection system as defined in any of the claims 5 to 11 inclusive where the pulse modulation represents two states and is of binary form.
13. A railway train detection system as defined in any of the claims 5 to 11 inclusive where the pulse modulation represents more than two states and is of multi state form (m-ary) with 'm' possible states where 'm' is an integer.
14. A railway train detection system as defined in any of the claims 5 to 11 inclusive where the pulse modulation represents a value of analogue form with a continuously variable state.
15. A railway train detection system as defined in any of the claims 12 to 14 inclusive where the track circuit signal is electrically separated from adjacent contiguous tracks by means of insulated rail joints (IRJs).
16. A railway train detection system as defined in any of the claims 12 to 14 inclusive where the track circuit signal is electrically separated from adjacent contiguous tracks by means of terminating bonds connected across the track in conjunction with simple tuned areas formed by capacitor tuning the inductance of a section of the track circuit abutting a terminating bond.
17. A railway train detection system as defined in any of the claims 12 to 14 inclusive where the track circuit signal is electrically separated from adjacent contiguous tracks by means of a compound tuned area, part of which includes passive components included to form a low impedance path across the track at the track circuit carrier frequency in conjunction with capacitor tuning the inductance of a section of the track circuit abutting the low impedance path.
18. A railway train detection system as defined in any of the claims 12 to 14 inclusive where the track circuit signal is electrically separated from adjacent contiguous tracks by means a combination of methods described in any of the claims 15 to 17 inclusive.
19. A railway train detection system as defined in any of the claims 15 to 18 inclusive where the receiver comprises two separate channels to check the integrity of the received signal.

20. A railway train detection system as defined in claim 19 where the separate channels within the receiver use diverse means of establishing the content of the received signal.

21. A railway train detection system as defined in either of claims 19 or 20 where the separate channels within the receiver are alternately energised in sequence to provide a valid track clear signal.

22. A railway train detection system as described in any of the preceding claims where the absence of a valid receiver signal indicates a possible rail fracture within the track circuit.

23. A railway train detection system as described in any of the preceding claims where part of the coded signal is used to communicate information from the transmitter to the train.

24. A railway train detection system as described in any of the preceding claims where the peak value of the signal across the track under track unoccupied conditions is in excess of 20V.

Amendments to the claims have been filed as follows Claims 1. A railway track circuit for detecting the presence or absence of a railway train within a region of railway track monitored by the circuit, the circuit comprising a receiver and a transmitter connected via a section of the railway track to the receiver, the transmitter being arranged to apply signals to the railway track, the receiver being arranged to receive and detect a transmitted signal from the transmitter via the railway track, so that the circuit may deem whether a train is present or absent within the monitored region in dependence on the received signal, wherein the transmitted signal is in the form of pulses representative of a code, and the elements of the code are represented by time intervals derivable from the pulses, whereby, in use when a train is present in the monitored region, the attenuation of the transmitted signal due to the shunting effect of the train is detected by the receiver resulting in the circuit deeming a train present.

2. A railway track circuit according to claim 1 wherein elements ofthe code are represented by time intervals, each time interval being a time interval between successive pulses so that the pulses are coded using pulse position modulation.

3. A railway track circuit according to claim I or claim 2 wherein elements of the code are represented by time intervals, each time interval being a width of a pulse so that the pulses are coded using pulse width modulation.

4. A railway train detection track circuit comprising a transmitter and at least one receiver connected via a section of railway track used to ascertain the presence or absence of a train by establishing the attenuation of the transmitted signal as a result of the shunting effect of train axles where the transmitted signal applied to the track is in the form of pulses modulated with coded pulse position modulation.

5. I\ railway train detection track circuit comprising a tra Emitter and at least one receiver connected via a section of railway track used to ascertain the presence or absence of a train by establishing the attenuation of the transmitted signal as a result of the shunting effect of train axles where the transmitted signal applied to the track is in the form of pulses modulated with coded pulse width modulation.

6. A railway Pack circuit according to any previous claim wherein the pulses are coded with a combination of pulse width modulation and pulse position modulation.

7. A railway track circuit according to any previous claim wherein the pulses are of unidirectional polarity.

8. A railway track circuit according to any of claims I to 6 wherein the pulses are of varying polarity.

9. A railway track circuit according to any previous claim wherein the pulses are represented by an alternating signal having a frequency in the range from 200Hz to 20kHz.

10. A railway track circuit according to any previous claim wherein the pulses are represented by alternating signal bursts at a single frequency.

11. A railway track circuit according to any of claims l to 9 wherein the pulses are represented by alternating signal bursts at more than one frequency.

12. A railway track circuit according to any of claims I to 9 wherein the pulses are represented by a change in frequency of a carrier signal to produce a frequency shift keyed (FSK) signal.

13. A railway track circuit according to any of claims I to 9 wherein the pulses are represented by a change in phase of a carrier signal to produce a phase shift keyed (PSK) signal or a differential phase shift keyed (DPSK) signal.

14. A railway track circuit according to any previous claim wherein the width of each pulse, or the time interval between each pair of successive pulses, is representative of one of a plurality of possible states, the pulses being representative of a code of multi state n-ary form, where 'n' is an integer greater than 1.

] 5. A railway track circuit according to any previous claim wherein the width of each pulse, or the time interval between each pair of successive pulses, is representative of one of two possible states, the pulses being representative of a code in binary form.

16. A railway track circuit according to any of claims l to 14 wherein the width of each pulse, or the time interval between each pair of successive pulses, is representative of one of a multiplicity of possible states, the pulses being representative of a code of multi state wary form where 'm' is an integer.

17. A railway track circuit according to any of claims 1 to 14 wherein the width of each pulse, or the thee interval between each pair of successive pulses, is representative of a value of analogue form.

18. A railway track circuit according to any previous claim wherein the track circuit is electrically separated from adjacent contiguous tracks by means of insulated rail joints (IRJs).

l9. A railway track circuit according to any of claims I to 17 wherein the track circuit is electrically separated from adjacent contiguous tracks by means of terminating bonds 'I connected across the track in conjunction with simple tuned areas formed by capacitor tuning I the inductance of a section of the track circuit abutting a terminating bond.
20. A railway track circuit according to any of claims 1 to 17 wherein the track circuit is electrically separated from adjacent contiguous tracks by means of a compound tuned area, part of which includes passive components included to form a low impedance path across the track at the track circuit carrier frequency in conjunction with capacitor tuning the inductance of a section of the track circuit abutting the low impedance path.
21. A railway track circuit according to any of claims 1 to 17 wherein the track circuit is electrically separated from adjacent contiguous tracks by means of a combination of the means described in any of claims 18 to 20.
22. A railway track circuit according to any previous claim wherein the receiver comprises two separate channels to check the integrity of the received signal. l]
23. A railway track circuit according to claim 22 wherein the separate channels within the receiver use diverse means of establishing the content of the received signal.
24. A railway track circuit according to claim 22 or 23 wherein the separate channels within the receiver are alternately energised in sequence to provide a valid track clear signal.
25. A railway track circuit according to any of the previous claims wherein the absence of a valid receiver signal indicates a possible rail fracture within the track circuit.
26. A railway track circuit according to any of the previous claims wherein part of the coded signal is used to communicate information from the transmitter to a train.
27. A railway track circuit according to any of the previous claims wherein the peak value of the signal across the track under track unoccupied conditions is in excess of 20V.
28. A method of detecting the presence or absence of a railway train within a region of monitored railway track, the method comprising the steps of applying a signal to the railway track, the signal passing via a section of the track and then being received, detecting from the received signal the amount by which the signal applied has been attenuated and deeming whether a train is present or absent within the monitored region in dependence on the attenuation of the signal received, wherein the applied signal is in the form of pulses representative of a code, and the elements of the code are represented by time intervals derivable from the pulses.
29. A kit of parts suitable for producing a railway train detection circuit, the kit comprising a receiver and a transmitter, the receiver and transmitter being so configured as to be suitable for use as the receiver and transmitter of the railway track circuit according to any of claims 1 to 27.
30. A kit of parts according to claim 29 wherein the kit of parts is configured so as to be suitable for converting a section of existing railway track into a railway track circuit according to the railway track circuit of any of claims 1 to 27.