Interesting post by KRK. Also there are several ways to get the light up and done both vertically and horizontally. As cab height is pretty well fixed and signal hight my be with single, double or triple heights. In an H2 GS&S for example the light is fixed and focused (perhaps the signal is located on a curve). The colors are in a pearshaped drop with the colored glasses. The pearshaped drop piviots at the top and moves the desired color into the light beam. The signal head itself can be adjusted up and down and side to side. The light lens cover also can have groves and ridges that focus the light in desired ways...for example 15 degrees. It can be mounted in the lens to focus the light anyway desired. Signal manuals explaining all these signal functions can be found online.
Roger B