With temperatures predicted to soar to an unprecedented high of 39C (102F) today, many train companies in the eastern region are urging people not to travel unless absolutely necessary.
Network Rail has announced that speed restrictions will be in place for most of the day because of fears that rail tracks could buckle.
Thameslink, Great Northern and Southern have said there will be delays and cancellations to services across the whole network.
East Midlands trains are not calling at Bedford and Southern trains to Milton Keynes are not running.
Greater Anglia has advised people against travelling unless its urgent.
John Halsall, from Network Rail, said: "I've been working on the railway for 10 years - I've never encountered temperatures like this or put in the level of blanket speeds that we're talking about.
"We're not making this decision easily but it is essential in terms of passenger safety."