Like most UK lines, the Ffestiniog once used bullhead rail (rail with a rounded head and foot, held in chairs rather than spiked down) but it's used flat-bottom rail rolled to its own specification for some time now - so it's just a combination of the light and the fact that the ballast is well up to the rail-head.

Bullhead is still alive and well in some applications - the London Underground still use it exclusively, I think.

From the modelling / weathering point of view, what I find fascinating about the photo is that this very smartly-turned-out loco is still rather tatty in places - note the way that the lining, and indeed the whole paint-scheme, disappears around the front of the cab behind the dome.

You can also (just) see the twin regulators in the cab; the levers are painted red and the handles for the two engines (one loco, two engines!) sit right next to each other, making it very easy for the driver to control them either separately or together.

John