About 20 years or so ago, when travelling on the Night Riviera most stations had their lights switched off when passing through in the early hours of the morning but recently I noticed that all stations now seen to be lit (although many towns now switch off street lights overnight).  Is everywhere now lit through the night or is lighting triggered by the approach of trains?

This would only work with LED lighting, and then only if the lights are specified as being dimmable.
Sodium or metal halide lamps cant realistically be dimmed, fluorescent and compact fluorescent lamps can only be dimmed at considerable cost and complication. Incandescent lamps are readily dimmable but little used on account of energy wasted.

Has this always been a requirement?  When I did an overnight trip in 2002, the FGW^▸ (First Great Western) (only intercity back then) stations were lit, Wessex were dark and Thames were lit.  This was before councils started switching off street lights overnight though so although I could not see the Wessex stations we passed through, I was still able to work out where we were.