Today's high temperatures led to speed restrictions on a busy rail commuter route.
Speeds were reduced from 90mph to 80mph - and were later expected to go down to 60mph - on the London to Norwich Great Eastern line operated by the National Express East Anglia train company.
A Network Rail spokeswoman said: "We're already doing weekend work on overhead wires on this route and the wires can overheat in very hot weather.
"A speed restriction has been introduced and it may be lowered to 60mph in the hottest part of the day."
National Express East Anglia services were expected to be affected until 5pm today.
Between 12 noon and 5pm trains between London Liverpool Street station and Braintree in Essex will terminate at, and start from, Witham. A revised train service will run between Witham and Braintree.
Service alterations will be made on other East Anglia routes to and from Ipswich, Clacton-on-Sea, Colchester and Southend Victoria.