376,913. Railway signalling systems. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Arkenburgh, W. H. ; 1135, Parkwood Boulevard, Schenectady, New York, U.S.A.) June 5, 1931, No. 16481. Convention date, June 6, 1930. [Class 105.1 A railway signalling system in which a line conductor extends the length of each block section and is connected with one of the rails to form a loop circuit is provided with means whereby the line conductors of adjacent blocks are connected together to form with the corresponding rails of the adjacent blocks a single loop circuit when a train lies in both the blocks. Speech communication is therefore always possible between the front and rear of the train by inductive transmission and reception over such loop circuit. The Specification also describes' an automatic signalling system for giving an indication of the number of sections a train ahead is distant and of its speed both to a following train or to a fixed station by means of a transmitter on the train which energizes the loop circuit extending the length of the occupied block section with current of one frequency which at the rear end feeds a frequency-changing relay whereby current of a different frequency is supplied to the rearward loop circuits. The transmitter current may be modulated by an axle-driven interrupter 23, Fig. 3, so that an indication of the train speed may be given on this train by a device 24 and on a following train by a device 25 responding to all the changed frequency currents and giving an indication dependent on the rate of interruption ; other devices 26, 27, 28 selectively responsive to the different changed frequency currents give an indication of the number of sections the train is ahead. Speech communication on the train or with a station is effected by transmitting to the loop circuit of the occupied section microphone-modulated current of a different frequency from a device 30. The speech and signalling currents are transmitted and received by coils 22 arranged in a plane parallel with the track rails one rail being connected to a line wire by relay contacts to form the loop circuit. When the train lies across the junction of two sections the adjacent loop circuit line wires are connected by relay contacts so that one loop circuit is formed extending over the two sections. Fig. 5 shows the arrangement of the circuits along the track. A number of the loop circuits may be coupled as by transformers 11<1> with a line 38 leading to a fixed station 39 so that speech communication can be established while the train is passing through a plurality of sections. Each block section such as B includes; a track relay 29 which is fed from a transformer secondary 31<1> and controls over contact 34 a line relay 32<1> at the other end of the section ; a frequency changing relay 16 comprising amplifiers a, b, c which respectively respond to input current of frequencies f<111>, f<11>, f<1>, and provide relayed current of frequency f<11>, f<1>, f and a repeating amplifier d which responds to current of frequency f ; and relays 371, 421 to connect the line wires 61, 6<11> of the loop circuits of block sections B, C during the passage of a train over the junction of the sections. Blocking circuits 47 to reject the power current frequency, chokes 48 to reject the signalling current frequencies, and tuned acceptor circuits 41 for the speech current frequency are provided. With a train in block section B the track and line relays 29, 321 will be de-energized. The oscillations of speech-current frequency transmitted by the train coils 22 to the rails will flow in the circuit: upper rail, wire 40, back contact of top armature of track relay 29, speech-current acceptor circuit 41, wire 36, front contact 8 of relay 37, loop circuit line 6<1>, front contact 7<1> of relay 37<1>, back contact of top armature of line relay 32<1>, back to the upper rail. With a train extending into both sections B, C over the junction of the sections the track relay 29<1> of section C will be de-energized and at its lowest armature will open the circuit from transformer secondary 33<1> supplying the relay 37<1>, The dropped armatures 7<1>, 8<1> then connect the loop circuit lines 61, 611 and a single loop extending over the two sections B, C is thus formed and so permits communication between a rear vehicle of the train still in section B although the locomotive is now in section C. The double length loop is not however formed if a prior train already occupies section C before the entry of the train into section B since the relay 42<1>, which normally is energized from transformer 33<1> over front contact of the lowest armature of line relay 321 and normally therefore is de-energized when a train enters section B, is now maintained energized by a stick circuit over back contact of the lowest armature of the track relay 291 of section C ; the front contact of lower armature of relay 42<1> now maintains the relay 37<1> energized and therefore prevents the joining up of the two loop lines 61 and 6<11>. For the block signalling giving an indication of the number of sections a train ahead is distant the locomotive apparatus impresses on the loop circuit of the occupied section, say section D, a signalling current of frequency f<111> which, being blocked by the speech-current acceptor circuit 41<11>, energizes the frequencychanging amplifier a<11> whereby current of frequency f<11> from the output side thereof passes to the loop circuit of the rear section C ; this circuit is from the lower terminal of the amplifier all, over wire 4411, upper rail of section C, wire 40<1>, front contact of top armature of track relay 29<1> to the input side of amplifiers b<1>, c<1>, d<1> (only b<1> responding to the frequency f<11>), and thence over wire' 36<1>, front contact 8<1>, loop circuit line 6<11> of section C, front contact 7<11>, front contact of top armature of line relay 32<11> of section C, back contact 45<11> of track relay 29<11> of section D, to the upper terminal of amplifier a<11>. The loop circuit of section C therefore carries signalling current of frequency f<11>. -Similarly section B carries loop current of frequency f<1> derived, from the frequency changing amplifier b<1> ; section A loop current of frequency f derived from frequency changing amplifier c ; and all sections to the rear carry loop current of the same frequency f relayed by repeater amplifiers d. An overtaking train in section A by deenergizing line relay 32 and dropping its second armature will connect that amplifier output terminal which normally leads to the section A loop line 6, to the lower rail of section A. Accordingly the train by bridging the rails prevents such signal current proceeding further and can receive the signal by means of the coils 22 ; the receiving device 28 responds to frequency f and indicates that the ahead train is at least two intervening sections distant while the device 25 responding to all frequencies gives an indication of the rate of interruption and therefore of the speed of the ahead train. If the overtaking train is in section B the received frequency is f<1> to which the device 27 responds and indicates that there is one intervening section. In section C the received frequency is fll to which the device 26 responds and indicates that the section ahead is occupied. The receiving devices do not respond to the frequency f<111> of the train transmission generator. The receivers 26, 27, 28 may be of the form described in Specification 352,017, [Group XL], and the frequency-changing relays a, b, c may be of the electron discharge frequency multiplier or of the heterodyne beat frequency changer constructions. The stations 39 may likewise be provided with devices which respond selectively to the different frequency signal currents and thereby indicate the position of the train along the track.