Do you remember white walled tyres? Well.....

[Kromaatikse]

Well, the part that has been painted has, judging by the wobbliness of the alignment, been known to shift and buckle in the past. Presumably there is some weakness in the formation which is not readily apparent from a casual glance - perhaps it gets damp in that leafy hollow? The part that hasn't been painted looks to be in better condition.

On a low-traffic line, it is much cheaper to apply some paint to reduce the stress on the formation than to excavate and rebuild the formation to a higher standard. This intense minimisation of maintenance costs is also probably the reason behind the incredibly low speed limit. There might be a question of why welded rail is used rather than jointed rail, which would have less susceptibility to thermal expansion - but jointed rail needs more frequent examination from ground level because it has more moving parts.

As for the effectiveness of the paint, as you say the thermal conductivity of steel is high, so even though it is only in the sun for half the day, that half-day *matters*. The top of the rail will be polished to some extent by the passage of even slow trains. The web of the rail has been painted - the thinnest part - leaving only the small surface area of the side of the rail head able to absorb significant energy.

[loverswalk]

We've got a diamond crossing painted white at Brighton where the line from Platform 2 crosses over the up and Down West Branch as it access Lovers Walk, I've seen the white paint on the side of the rails as I walk along platform No2 on my home and it had me puzzled until now.
LW