Trivial really but probably a good explanation

[oldragbag]

Traffic lights are from top to bottom, red yellow (amber) and green. Why are railway colour light signals the opposite way round with red at the bottom etc.?

[NazNomad]

To avoid confusion between the two ... that simple. ;-)

[Scylax]

Ah, but did you know that red means stop, green means go, and amber means go faster?

[heathfield]

Having a red road traffic light at the top means it can be more easily seen at a distance, and especially over a line of queueing traffic.
The red light on railways is lower to be more within the driver's line of sight and provide maximum brightness through the lens. (The lens is normally focussed on a point 200 yards up the line, whereas road traffic signals have no focussing lens). There will be no queueing railway traffic to see over, since the 'block' the train has entered must be clear before such entry would be allowed.