Signal Mast Logic - The Layout
The track arrangement chosen for the tutorial is shown in the illustration below. Being a UK prototype, trains run on the left hand line of a double track. In usual UK terms, the "Down" line is the direction furthest from London, and the Up is heading towards London.
Our layout has a double track through line, and a terminating Bay platform. Trains can enter the Bay from the down line. Trains can exit the Bay platform, crossing over the Down line to the Up line.
The signals shown are three-aspect colour light, which in UK arrangements show red for danger (stop), yellow for caution (pass, but expect to stop at next signal) and green for proceed (next signal will be either green or yellow). The signal which controls entry to the Bay has a junction indicator (feather) which will illuminate when the route to the Bay is set. This signal also has a calling-on light, or shunt signal, in the form of two white lamps below the main signal. These lights are used to permit a train to pass the signal even if the track ahead is occupied, for example a locomotive might move onto the occupied Bay to allow it to couple to stock in the Bay.
There could be other signalling on this track plan, for example, shunt (ground) signals to control the cross-over for traffic moving between the Up and Down lines, but we felt that the arrangement in the diagram has sufficient features for the tutorial.