When exposed to temperature variations, the rail tends to vary its length. If this tendency would be freely allowed, for a temperature variation Δt° the rail length L will vary by ΔL.
This length variation is ΔL = αLΔt°.

If, on the other hand, the expansion is not permitted in any way, and both ends of the rail are fixed, the thermal variation Δt° will generate in the rail a compression axial (normal) internal force, N.
This axial internal force is N = αEA Δt°.

In these formulas, α is the steel expansion coefficient, E is the steel elasticity modulus and A is the rail section area.

One way to reduce the influence of this temperature variation is to reduce Δt° itself, consequently reducing the caloric energy absorbed by the rail. A simple and effective way to do this is to paint the rails white. In this way a significant percent of the infrared light is reflected and the temperature of the painted rail is lower of the not-painted one.

An AREMA research on this subject showed that, depending on the coating used, the rail temperature reduction can be up to 10°C:

Based on Ritter, G. W., Al-Nazeer, L. (2014). Coatings to Control Solar Heat Gain on Rails. AREMA 2014 Conference

The method is efficiently used in UK and around the world. Network Rail is mentioning it in its video about buckled rail.

Here are a few pictures I took on the Italian lines where, at least in the Milan area, all the main lines are continuously painted white:

References

• Ritter, G. W., Al-Nazeer, L. (2014). Coatings to Control Solar Heat Gain on Rails. AREMA 2014 Conference.