NR rules

My recollection is that LUL have an exemption as LUL trains do not reach the high speeds of most main line stocks. But train fronts must be at least 50% red. Not sure that the all white of R stock would now be allowed. The middle cabs of some 1972 stock now have a half red front to allow for their use as pilot cars taking units to Acton Works from Stonebridge Park and then returning with the otherwise all white middle DM car at an outer end.

That said, I presume some sort of exemption also applies to such likes as the running of steam trains or Sarah Siddons if on NR ?

There are all sorts of rules on NR that LU don't conform to. NR sets rules to cover 20,000 miles of main line. District works over about 2.5miles of it. the main arguement about red and yellow fronts is that both are irrelevant if you have two reasonably powerful headlights - hence it applied for and obtained derogations for the 1972 stock, C and D stock - and is doing similar for S stock.

In addition, as LU never exceeds 60mph and routinely nips in and out of tunnel, it doesn't see the need to conform to the NR rules for day and night headlights (the reason why they have two but only one is on at any time). Day headlights are VERY bright!

The yellow is, these days, a back-up in case the headlight fails during a journey. LU manages this risk with two headlights and (by NR standards) short trips.

NR also allows others to have this dispensation, especially for historic/steam trains subject to operating less than 10,000 miles a year, so Deltics and Flying Scotsman aren't disfigured with yellow fronts.