Many were trapped on sweltering trains with no air-conditioning while others as temperatures soared into the mid-30s.
And others were locked out of London Euston station as Network Rail attempted to as the concourse became rammed with overheating stranded passengers.
One furious traveller tweeted: "Not only have we been stuck on this service for 2hr 20mins on the hottest day of the year, there’s no water on the train, half-hearted “updates” from National Rail, and no end in sight in terms of ETA."
Another ranted: "Nearly three hours sat still on the track, no water, no windows, no air-con, no working toilets!"
Network Rail reported damage to overheard power cables between Leighton Buzzard and Bletchley in mid-afternoon affecting London Northwestern Railway and Avanti West Coast services, just as people were gearing up for the Friday commute home.
Operators’ customer service teams were swamped with passengers struggling to find a way home and some were advised to use Stagecoach buses between Milton Keynes and Wellingborough to connect with East Midlands Railaway services to and from St Pancras.
Trains began being cancelled from 4pm, passengers were initially told to expect “major disruption” until 6pm, then 10pm and then until the end of Friday's service.
Euston station finally reopened just after 7pm but with severely limited services running to get people home.