With the heatwave expected to peak on Thursday and Friday, several c2c trains have been suspended for the next two days, while East Midlands Railway has advised passengers to avoid all but essential travel.
West Midlands Railway and London Northwestern Railway reported “heat related issues” affected some of their services early on Thursday.
Operators South Western Railway and Great Western Railways have warned they are expecting to face similar disruption.
Transport for Wales has said a speed restriction is in place due to high track temperatures between Aberdare and Fernhill in South Wales, with disruption to services expected until 21:30.
Separately, the UK’s electricity grid operator Neso has issued a request for more power to be made available during the ongoing heatwave.
Extreme heat can make energy processes less efficient and lead to a spike in electricity demand as people increase their use of air conditioning, freezers and fridges.
Neso’s notice warns there could be “tight electricity margins” on Thursday evening, but does not suggest a risk of shortages or blackouts.
Meanwhile, the health service in England has asked the public – particularly children, the elderly and those with existing conditions – to “take the heat seriously”, as it warned many of its ambulance services were under pressure.
National medical director Prof Frankie Swords urged others to look out for those who might be vulnerable.
The National Fire Chiefs Council (NFCC) warned on Wednesday that the dry and hot weather posed an increased risk of wildfires. The NFCC also advised people to avoid using disposable barbecues in open countryside, park and moorlands.
While the worst heat is expected over the next two days, temperatures could remain well above the July average of 18-23C for another week – which would make it one of the UK’s longest heatwaves since 1976.
It comes weeks after the hottest June ever recorded across western Europe – and the second-hottest seen globally – when UK temperatures peaked at a record-breaking 37.7C in Norfolk.