Signalling System Outline

1.0 Signal Plant
A signal system for a model railroad consists of a series of Signal Plants.  A Signal Plant consists of a series of components and, depending on the type of plant, may include:

• Occupancy Detectors which detect whether an electrically isolated section of track (a block) is occupied by a train.  The detection method can be infrared, light interruption, or current sensing.
• Turnout Direction Indicators - a system of switch contacts mounted on the turnout or on the switch machine that determine if a turnout is thrown for the "mainline" (straight-thru) or the "diverging" route.   
• Signal Control Boards (SCBs) which take information from the Occupancy Detectors, Turnout Direction Indicators, and other Signal Control Boards.  Using this information, the SCB determines whether the signal masts that it controls should display red, yellow, or green.  (The SCB may also turn off all of the LEDs in its control if there is no train occupying the track that it controls.)
• Lineside Signal Masts located alongside the track display lights (LEDs) "telling" the train whether to stop, slow down, proceed, take the main line route or the diverging route of a switch.  (In our example, the lights are simply for display and have no control over the train.)  These different signal aspects (red, yellow, or green) are determined by the controlling Signal Control Board connected with that Signal Plant.  
• Wiring System - cables or wires that physically connect the components together and transmits information to/from the Occupancy Detectors, Turnout Direction Indicators, and Signal Control Boards so as to illuminate the LEDs in Trackside Signal Masts in red, yellow or green. 

(Insert graphics of each of the above)


2.0  East-West, Mainline Route - Diverging Route
The following conventions will be used when looking at trackwork within a signal plant.

2.1  East End - West End
An insulated section of track has two ends - the East End, and the West End.  A train entering the block is either an Eastbound train heading East or a Westbound train heading West.

2.2  Turnout Routes
Trains can go through a turnout in two directions - the Mainline Route, and the Diverging Route.  The Mainline Route is that part of the turnout which obviously takes the train through the mainline mainline.  It has two parts - the mainline entering route (MAINENTER), and the mainline continuing route (MAINCONT).  The diverging route is that part of the turnout that curves away from the main line. 

Turnout Plant

3.0  Types of Signal Plant
There are two types of signal plants

• Block plant
• Turnout plant

3.1  Block Plant
The block plant is the simplest type of plant in the signaling system.  It consists of:

• An electrically-insulated section of track
• Occupancy Detectors that determine whether a train is on that section of track
• A Signal Control Board (SCB) that processes the occupancy information on that section of track and information received from adjacent signal plants.  It then lights up the signal masts under its control according to the information it has processed.  It also passes its information on to the adjacent Signal Plants. 
• Lineside Signal Masts located at the EAST and WEST end of the boundary for that section of track. 

Block Plant

3.2 Turnout Plant
The Turnout Plant consists of

• A Turnout that will route a train through the Mainline Continuing Route (MAINCONT) or onto the Diverging Route (DIVERGE)
• Turnout Direction Indicators that indicate whether the turnout is thrown for the Mainline Continuing (MAINCONT) Route or the Diverging (DIVERGE) Route
• Signal Control Boards
• Lineside Signal Masts

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