597,526. Electric selective signalling. WESTINGHOUSE BRAKE & SIGNAL CO., Ltd. May 28, 1945, No. 13307. Convention date, Sept. 2, 1944. [Class 40 (i)] In an electrically operated centralized railway traffic control system of the coded typed, e.g. as described in Specification 415,787, the single communication channel between the central office and stations, is divided into two or more sections interconnected by two-way repeaters, the direction of transmission through a repeater being determined in accordance with the length of the first step of the received code. A key is also provided at the central station, whereby transmission from a remote station to the central office may be stopped. The line sections are separately energized, thereby enabling longer lines to be used, and currents of different character may be used in the different sections. Carrier currents, e.g., may be used in some sections which may be wired, or form radio links. Fig. 1 shows a single repeater station giving two-way communication between a central office and a field station over line sections 1, 2. The apparatus at the field station and at the office is as disclosed in Specification 415,787, and includes a master relay M, OM, operation of which initiates transmission of a signal, transmitter relay T, OT, coding unit CU and line relay R, OR. The repeater comprises line relays R1, R2, a timing or slow acting relay L and stick relays 1LB, 2LB. The line relays are energized, the relays OR, R1 from the battery B1 at the office and relays R, R2 from battery B2 at the repeater. Initiation of a signal from the office or the station of energization of relay OM or M causes the corresponding transmitter relay OT, T under control of its coding unit CU to interrupt periodically line 1 or 2 as the case may be. In either case the opening of one line interrupts the other and relays R1, R2 both release. The slow-acting relay L is then energized from source B over contacts a of relays 1LB, and as L picks up, relay 1LB is energized 'over contact a thereof. At its contacts a relay 1LB de-energizes relay L and at its contacts b bridges the b contacts of relay R2 to prevent transmission from line 2 to line 1, thereby establishing one-way transmission from line 1 to line 2. If the code signal has been initiated at the office, the first code step corresponding to release of R1 is a relatively long one and relay L releases before the second step of the code is reached. Relay R1 now repeats the code received over line 1 into line 2 to operate relay R at each station correspondingly, the coding units at the stations being conditioned by the first long step to receive the remainder of the code as explained in Specification 415,787. Relay 1LB is held picked up by its intermittent energization over contact a of relay R1 on the odd numbered code steps and releases when R1 becomes steadily energized at the end of the transmission, thereby restoring the repeater to normal. Should a signal be initiated at a station and not at the office, the first code step as repeated into line 1 terminates as soon as relay 1LB picks up to close its contacts b and energize time relays R1, OR. Relays R2 and R pcik up as soon as relay T releases to begin the second step of the code. Since in this case the first code step is relatively short, R2 and R pickup before L has released, thereby energizing relay 2LB over the a contacts of relays R1, R2 and b contacts of relay L. Relay 2LB thus picks up on the second code step of a station signal and at its b contacts removes the shunt across contact b of relay R2 so that R2 may repeat code impulses into line 1 from line 2. At the same time contacts c, d of relay 2LB render relay R1 ineffective to control line 2, thus enabling relay R2 to repeat the code received from line 2 into line 1 to operate office line relay OR in accordance with station transmitter relay T. During this transmission, relay 1LB is held picked up, as before, by intermittent energization on the odd-numbered steps and 2LB is held picked up over contact c of 1LB by intermittent energization on the even numbered steps of the code until R1 becomes steadily energized at the end of transmission. Should relays OM, OT be energized to initiate a control code signal from the office at the same time as a field station signal is initiated or during the transmission of the first step of such a signal, relay R1 remains released, holding line 2 open after relay T releases to cause relay R to respond to the first long step of the control signal and conditioning the station coding unit to receive the office control signal, thereby preventing further operation of relay T until the code signal has been completely transmitted. The control station thus takes precedence in transmission provided the master relay OM thereat has been operated. Disabling a field station. Depression of a push-button PB at the central office steadily energizes relay OT, thereby opening lines 1, 2 and releasing all line relays to stop transmission of a station signal, should one be in course of transmission. Plurality of field stations. Each line section 1, 2, may include a plurality of field stations connected thereto, as described in Specification 415,787. Should the field station initiate a signal simultaneously, the operation of relay M at the station nearer the office opens the line leading to the more remote station so that the two stations transmit successively without interference as disclosed in the above-mentioned Specification. Modifications. In another embodiment, Fig. 2, the two-way transmission over line 3 between the office and the repeater is by carrier current derived from oscillators of the same frequency located at the office and repeater. Transmission between the repeater and the field stations which are connected to line 4 is by pulses derived from battery B4 at the repeater and B4a at a station. The line sections are normally de-energized, the line relays OR R, ... being energized on the odd steps of each signal code in contradistinction to the arrangement of Fig. 1, and low-pass filter LP between each station and line 4 render this line available for voice' or carrier current communication. The repeater is similar to that of Fig. 1, but includes delivery of carrier-current codes to tine 3 from the repeater. A plurality of line sections 4 may be provided, each terminating at one end in a repeater, transmission between the respective repeaters and the central office being effected at different carrier frequencies. In another modification, Fig. 3 (not shown), two battery energized multiple connected line sections, similar to those of Fig. 2 are connected through repeaters to a single frequency two-way radio channel.