The heat has resulted in tracks expanding, meaning trains can only run at a slower pace.
(Permalink) Heat Rail buckling ScotRail

Network Rail has been preparing to keep passengers moving throughout the next few months. Rails in direct sunlight can be as much as 20C hotter than air temperatures and expand as they heat up, causing them to curve or buckle. Engineers have been stressing sections of track (artificially stretching the rails) in known hot-spot areas to help them cope with sudden rises in temperature and painting the rails white in key locations to reflect the sun, keeping them up to 10C cooler and helping prevent buckling.

Overhead electric power lines can also expand in prolonged heat, causing them to sag, which can disrupt train services on busy routes as speed restrictions need
to be introduced to prevent trains snagging on the wires. To combat this, the tension in the wires at some locations has been adjusted to levels that will help prevent sagging and keep trains running.

Remote temperature monitoring equipment has been installed on rails at known hotspots allowing decisions on whether to implement speed restrictions to be made in real-time when necessary for safety reasons.
(Permalink) Heat Network Rail OLE Rail buckling Remote temperature monitoring equipment