CTC for Dummies


Centralized Train Control

CTC is often spoken about but many people have only a vague idea of what it is.
Basically it is a system for remotely controlling the 'Switches' and signals at a remote station or crossing loop. Integral with this is the setting of the signals associated with the points. turnouts or 'Switches' and the checking of nearby track conditions. It is more than toggle switches on a control panel with wires going out to each individual 'Switch' or signal.

It will be used to set a route through several station and loops according to the timetable or to control train crossings at loops.


The CTC controls many stations and communicates over a single cable connected to all. Control codes are sent to the loops and stations with commands for the 'Switch' position and the signals. An acknowledgment is sent back when the commands have been executed.

The station or loop (called a Field set) is connected to the track and signals. Tracks occupied, signal positions, train passage detectors are all reported back to the Field controller.

Before a command is executed the local conditions are checked and if correct the command is processed. These conditions are the 'Interlockings'

There is communications to the stations either side and these will send train advice to say a train is coming and occupying the track if single track working. On double track it would indicate that a train was on the block nearer it.


CTC panel

Each 'Switch' and the associated signals is controlled by 3 or 4 switches, usually set in a column on the CTC control panel. Two columns will control a station or loop. One for each "Switch".

These will control:

  1. The 'POINTS', a 2 position switch, either "N" (normal), that is straight ahead or "R" (Reverse) that is toward the loop tracks.

  2. The signal setting is either "L" , that is a train entering the station, or "R" for a train departing.

  3. The "Start" button is to send the codes to the addressed Field station. The "Code" light will indicate that a code has been sent and is being checked. This will extinguish when the Acknowledgment has been received.

  4. Other optional switches for control or permissions i.e. taking over local control (such as for shunting), secondary "Switches" on the main lines.

The Field set

This usually works automatic with only a switch to request local operations for shunting movements (or this can be done by telephone).

The track in the station proximity is divided into blocks and half blocks. There will be 4 to 8 blocks depending on track length, position of intermediate 'Switches' etc.

At the station or loop there are train detection circuits for:

  1. Each of the half blocks or blocks. Half blocks can be used to indicate train direction, how far away a train is etc.

  2. Train passage detectors are at various spots, beside signals, at the 'Switch', road crossings, shunting limits.

Signal setting:

The signals are interlocked by the above conditions. In the old manual signal box a signal could not be "cleared" (set to proceed) unless the associated "Switch" was set to that route.

A lot of terms still in use originate from the older systems.

In the model world.

Things are a little bit different. We are not miles away from the 'Switches', there are no station staff to organize the shunting.

Track control can be done from a central point using computer programs and track sensing accessories (DCC operation). There is a computer program called JRMI that can be used to control the layout and programmed for all stations. Interlock sensing is done with accessories decoders using the Loco Bus (Digitrak etc.) all input is on the computer screen.

If a real CTC panel is wanted and somewhat more prototypical operations, then the Field controller can be a specially programmed micro controller either at each station or at the CTC panel. Accepting codes from the CTC switches and checking the appropriate interlock and 'Switch' settings before setting the points and clearing the signals.

The diagram shows the logic needed for each station 'Switch' that the CTC panel communicates with.

Diagram of points logic