Complex sentence: Depending on the approach speed of trains, to a passenger it may seem that flashing aspects have failed when they have not, this is because the train passes the signal too quickly for a complete cycle to be observed from side-on.
Split into simpler sentences: Depending on the approach speed of trains, to a passenger it may seem that flashing aspects have failed when they have not. This is because the train passes the signal too quickly for a complete cycle to be observed from side-on.