Rail circuits

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Rail circuitry is a system in which, information is transmitted using the electric conductivity of track rails. Rail circuits consist of rail line, power source (works as transmitter), and track relay (works as receiver)

Rail circuits play major role in electrical centralization networks (Railway Switching Networks). They are used to determine whether a track is free or busy , and if the track is fully intact. They also serve as a channel to transmit data on Automated Locomotive Signaling devices.

1 Rail circuit action modes
2 Signaling and automatic blocking
3 Technical Details
4 Bibliography

[edit] Rail circuit action modes

Normal: Rail line is in working order, and free
Shunt: Rail line is busy by passing train
Control: Rail line is checked for fractures and cracks.

the structure of rail circuit

Rail circuit reaction to the above listed action modes is checked by change in signal voltage and phase. These changes are perceived by a discrete receiver. In Normal mode, the transmitter transmits "free" data signals. In shunt and control mode, it transmits "busy" signals.

There are two additional action modes of Rail circuitry.

ALS mode - The signal is transmitted for Automatic Locomotive Signals. They include a "traffic light" inside the locomotive cab. This light is supposed to repeat way light readings. Another very important feature is automated speed control.

Short Circuit mode - Comes in use to check the well-being of rail circuits, and signal power source. For this purpose the track is closed (shunted) at the powering end, by a passing train.

Rail circuits designed for controlling both main and side tracks of station, for non-stop train routing must work normally in normal, shunt, and control mode. The rail circuitry of stationary edge ways, and sorting hills do not encounter a control mode. Rail circuitry which is used to transmit signal to locomotives must support ALS mode. All rail circuits are obliged to check their power sources using a short circuit mode.

[edit] Signaling and automatic blocking

Automatic blocking is one of the basic means of regulating train traffic, and providing safe transport. For the purposes of automatic blocking, the railway stretch is divided into 1000 to 2600 meter segments. Every segment is fenced by signal lights. Signal changes, as well as segment blockage happen automatically while a train is in motion.

Code-based automatic blocking is an automatic blocking system which uses coded electric impulses as signals instead of a line/chain lead. The most common code is digital, where 3 consecutive impulses denote a green light; two consecutive signals denote a yellow light, and one impulse denotes a red light, etc.

[edit] Technical Details

Rail circuits can be classified by

their types,
carrier frequency of a signal current,
segment separation,
track receiver type, and
interference protection
double rail or single rail, traction return wise.

Commercial frequency alternating current rail circuits were highly popular. The phase sensing relay were in use (in receiver circuits). Later on when railway traction systems became electrified at 50 Hz, the rail circuits were rebuilt to 25 Hz, with continuous powering and another kind of phase sensing relay.
Later on, both on autonomous (diesel) traction, and direct current systems 25 Hz rail circuitry system came in use. If compared with 50 Hz systems, 25 Hz systems consume less power, work normally with lowered ballast resistance, have protection from traction and stray currents, and power voltage stabilization without phase regulation elements. Electrical separation of separate track segment is a very important problem, therefore low frequency circuits with segment separation are very popular.
There are some developments of tonal frequency rail circuits. This frequency ranges from 100 to 20000 hertz. They can work both with and without an isolating junction.

Jointless tonal circuits do not have exact borders, but are imprecise to the extend of about 10m.