Chiltern Railways cancelled its 4.37pm service to the West Midlands from Marylebone in Banbury at 5.38pm on bank holiday Monday.
Passengers reported the air conditioning had broken, with the sweltering conditions becoming unbearable inside the carriage.
A Chiltern Railways spokesperson said: “We are sorry that due to a fault with the air conditioning on a train, the 4.37pm service from London Marylebone to Stourbridge Junction on Monday, May 25 was cancelled at Banbury.
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“Customers were able to use their ticket to board the next service to the West Midlands which departed Banbury 50 minutes later.
“We encourage customers who were delayed as a result of the disruption to claim compensation via the Delay Repay scheme.”
The next service to the West Midlands departed Banbury at 6.27pm.
Staff handed out bottles of water to dehydrated passengers on Banbury’s platform.
The Daily Mail reported temperatures got up to 45C on the train.
Oxfordshire was one of nine counties where temperatures rose above 32C on Monday amid an historic bank holiday heatwave.
Records for May temperatures were broken at RAF Benson and RAF Brize Norton, where temperatures reached 33.6C and 32.9C respectively.
Until Monday, the highest May temperature had been 32.8C which was last reached in 1922 and 1944, the Met Office said.
