Here's an example:

I want to place a signal far enough behind (in this case North of) the chain rail signal at the bottom of this image so that there is enough room for a 4 car train (1-2-1) to fit between them, but the train car indicators are only shown behind the signal I want to place and not in front of it so I have to do a bit of eyeballing and guessing or mousing over the chain rail signal below to get it to show up.
The way it works now assumes you are only placing signals as you move forward along a track, but in my experience sometimes you need to move backward along a section of rail to place new signals - especially if you have just added a new intersection. Having the car indicators in front of the signal in addition to behind it will allow for proper spacing of signals to see if you can provide enough room to fit an entire train.