Faulty air-conditioning units on commuter trains across the west of England will not be fixed for months, a company has said.
Passengers on First Great Western's smaller trains said the warm weather is turning the carriages into heat traps.
They are fitted with air-conditioning units, but many do not work.
First Great Western said it is aware of the problem on trains it inherited from previous operator Wessex Trains, but that it will take months to repair.
The company said: "We've started a programme to repair and modify air-conditioning systems on these trains and this will continue throughout the summer.
"In the very high temperatures we've experienced this week, even those trains with air-conditioning can sometimes get warmer than we would like."