Fog and how it slows a train

Fog slows down a train because the driver cannot see signals until the last moment. Until a driver sees a signal, he has to assume it has the most restrictive aspect possible, based on the previous signal.

For example, suppose a driver passes a signal with a restrictive signal indication indicating that the next signal is at "stop". Further, suppose that as the train approaches this second signal, it changes from "stop" to one of the aspects allowing it to accellerate.

In clear weather, if the driver is within sight of the signal - even if still some distance away - he can immediately accellerate. In fog, however, he cannot see the signal until the last moment, which means he must drive slower longer.