Asymmetrical train front lights

[frankmoh]

Something I have always noticed about 465s (and 466s but for the purposes of this thread they are the same train) but never thought to ask why until now. Why are the lights on the front asymmetrical?

Despite having two circular front lights, the 465 usually opts for one circular light and one rectangular one. I have also seen 2 rectangular lights on with no circular ones.

And here's a picture of a Class 142 Pacer demonstrating the same strange lighting setup of one rectangular and one circular.

 

[CyrusWuff]

In short, for daytime running, trains are required to have one (traditionally the Driver's side) or both headlights lit in full beam mode, with other lights at the front being marker lights.

For night time running, this becomes both headlights in dipped beam mode plus the top marker lamp (where fitted.)

This is specified in Railway Group Standard GM/RT2131 "Audibility and Visibility of Trains."