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It is often desired to have some sort of electrical power distributed along the length of a train, whether that train is a load of scale cars, or dedicated passenger carrying cars. And another common requirement is communication between the guard and the driver. This is easier to achieve than at first thought - read on ...
It is assumed that a power source is available in the locomotive, or the driver's car. The power source is usually a battery of 12V (or sometimes 6V), and is located a) within the locomotive, or b) in or under the tender for a ride-on tender, or c) in the driver's car. This power is then distributed throughout the train by simple wiring, with a minimum number of wires.
Choice of Battery voltage - the most common voltage is 12V and there are small rechargeable batteries readily available. The preference would be 12V for the following reasons: 1) it is a common auxiliary voltage for 'diesel' or electric locomotives, 2) the voltage drop is smaller along the long lengths of wire required for a whole train.
The basic requirements for through-train wiring can be summarised below:-
The basic diagram is very simple - just 5 wires running the length of the train, with a simple straight-through jumper cable between each car. This arrangement is often called a bus or wiring bus. Devices are connected to the bus. Refer to Diagram 1 below:
Quite simple! And the above diagram is the minimum required. This is also the wiring diagram for the jumper cables between vehicles. The idea is that each car has the basic wiring bus, and uses only those connections needed for that car. The diagrams below show the various items and options possible. Diagram 2 below shows connections for a typical locomotive (or tender or driving car).
Power Source (Wire 1):- The most important item is the power source itself. This can be the 12V auxiliary supply in a diesel or electric loco, or a 12V service battery for a steam loco. This supplies all the necessary power to the entire train. Only 1 (one) power source is required per train.
Protection:- Of equal importance is a fuse or circuit breaker to protect the battery and the wiring from any serious damage in the event of a fault. Unfortunately, derailments do sometimes occur, and short circuit is possible. The only other thing required for the power is an ON/OFF switch or isolation switch.
Ground/Chassis (Wire 5):- The other essential wire required for the power supply. It provides the return path for any items connected to the wiring bus
Bell (Wire 2):- The bell provides 2-way communication between the guard and the driver. The bell (or buzzer) is mounted out-of-sight and in a position so the driver can still hear it when the train is in motion. To operate the bell, all that is needed is to connect the bell line to ground (chassis) anywhere along the train. Usually the guard has a bell push, and when operated the driver's bell sounds. By adding a bell push on the driver's car, it is now possible 1) to test the bell, and 2) communicate to the guard.
Train Continuity (Wire 4):- A desirable feature is a train continuity circuit to provide an alarm when a break-away is detected. The train continuity circuit uses the same bell as above. A continuity wire runs right through the train, and on the last car only, a shorting plug or small switch connects the continuity line (Wire 4) to the power supply (Wire 1). This energises a small relay which disconnects the driver's bell. If there is a break-away, continuity is broken and the relay drops out causing the bell to sound, thus alerting the driver. There is a Continuity Cancel switch to silence the alarm.
Lights (Wire 3):- It is now possible to control the lights within, or on, cars throughout the train from the driver's position. The lamps are simply connected from the Lights line (Wire 3) to ground/chassis (Wire 5).
Refer now to Diagram 3 below describing the various options possible on each car.
This section describes in more detail the various devices and wiring options that can be achieved.
Lights:- Lights are simply connected to the Lights (Wire 3) of the wiring bus. The lights are controlled by the driver (cab control. It is also possible to have the lights locally controlled by a switch within each car (local control). In this case the lights are connected to the Power Supply line (Wire 1) via the switch.
Another possibility is to have the car lights controlled by either the driver or locally. Note that either driver or local (or both) can turn the lights ON, but both must be OFF to turn the lights off.
Bell:- The guard's bell push is connected between the Bell line (Wire 2) and ground (Wire 5). Operating this bell push sounds the driver's bell. The guard's bell (connected between wires 1& and 2) is in parallel with the driver's bell and sounds at the same time. This allows the guard to have feedback that the bell is working, and also permits driver to guard communication as well.
Continuity:- On the last car only of the train (often the guard's car), there is a shorting link or switch from the Power Supply (Wire 1) and the Continuity line (Wire 4). This arms the continuity detector and makes it operational.
There is no particular connector specified, and this is left to individual circumstances and availability, and scale of the train in question. But what is important that the basic 5 wires of the bus are installed and connected to pins 1, 2, 3, 4 and 5 of the connector. What is important is that the connector is a firm fit and unlikely to work loose in normal use, but is able to be pulled out without damage under fault conditions (i.e. push-on, pull-off). (Connectors which screw into place provide excellent connections but virtually guarantee damaged wiring if they are pulled out in a derailment!). The connectors should be physically strong and robust, able to withsrtand normal wear and tear. Cheap flimsy connectors are not worth the effort and break quickly.
Occasionally, the full 5 wires of the bus are not required (say between a steam locomotive and its tender). As many wires as practical should be used, but the absolute minimum number is 2 wires (Power Supply  and Chassis )
[Updated: 20 Aug 2013]
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