In an article posted on the company's website, South West Trains explained why the heat was affecting their services.
Very hot weather can have an impact on the train service we can deliver. As with all metals, the track we use to run our trains expands and contracts depending on its temperature, i.e. the hotter it gets, the more it expands. Just like a car on a warm day, the metal can reach temperatures far in excess of the surrounding air temperature.
– South West Trains
Whilst we carry out a significant amount of work to prepare for the impact of hot weather by using hydraulic machines to artificially stress the rails to cope with high temperatures, over the weekend we have seen temperatures on our tracks of almost 50oc. The ageing condition of our infrastructure has meant that despite the preparation work that has taken place, we have had to impose speed restrictions at certain locations on our network