Three things would have to occur to allow such a collision: 1. track switch thrown to allow entry to the siding. 2. siding track energized in proximity to the parked train. 3. high speed of train.
Are you talking about a siding, controlled by a track switch at one end only? (A stub siding) You could wire the system so that the switch that allows entry to the siding is only allowed to be in the main line position when the siding is de-energized.
Whether or not you are electrically challenged, I'm sure you understand basic electricity (you know enough to wire a cut-off switch for a siding...right?) And I'm sure that you understand that a train can only proceed into a collision when it's directed into a siding and continues to get electricity.
To your original question: Yes, you can install a section of insulated outside rail in the siding, which will activate a semaphore (or any other signal) when the siding is occupied. But a constantly-operated semaphore will soon overheat and its coil will fail. Better to use just a block signal and/or a pilot lamp on your control panel. If you use a relay to activate the signal, additional contacts on the relay can be used to prevent electricity from getting to the switch coil for the siding choice. Since you already are using a (presumably) single-pole switch for the siding control, just substitute a double-pole switch, and use the extra contacts to de-energize the diverging track switch coil.
