Rail road signals and their placement

[railohio]

Specifically, I'm thinking of a modern light signal mast, that has two signal heads, each having red yellow and green lights. Let's say it's situated next to a double track main line, on which side would it most commonly be ? I asume this would be the right side, but I've also seen signals on the other side of the tracks. Which of the signal heads would be for the right hand track , the top or the bottom? And what if it's placed on the left hand side of the tracks?

Speaking very generally...

Signals with two heads stacked vertically are used at interlockings while signals with a single head are used as intermediate block signals. (Some intermediates have two heads when they are adjacent to interlocking signals.) Signals with two heads horizontally on a bracket are used by some railroads in double track territory instead of having signals on both sides of the tracks.

Signal placement will largely depend on the era and signaling system. By American standards signals are placed on the outside of double track territory. With full Centralized Traffic Control (CTC) trains can operate on any track in both directions without restrictions. As such signals appear in both directions for all tracks on the outside of the right-of-way.

For basic block signal operation (Rule 251) signals appear only for the designated direction on each track on the outside of the right-of-way. Trains operating against the current of traffic will require specific permission from the train dispatcher to do so.


Examples:

Single-head intermediate block signal facing the camera; double-head intermediate block signal facing train

Modern signals mounted horizontally on bracket for double track territory

Classic signals mounted horizontally on bracket for double track territory

Classic signals mounted on the outside of the right-of-way for double track territory


Further reading:

Railroad signals

Glossary of railroad signal terms

Railroad's traffic control systems