First Great Western has apologised after hundreds of passengers were left stranded on a broken-down train for almost six hours.
The company admitted the delay was "unacceptable" after a fractured air pipe caused the brakes to lock on its service from Penzance to London Paddington.
Just under 500 people were on board when it stalled near Pewsey, Wiltshire.
The train arrived at Paddington at around 10.15pm on Sunday - five hours and 40 minutes after its expected arrival time, First Great Western said.
Sue Evans, a spokeswoman for First Great Western, said: "I'm really sorry for the significant delay experienced by our customers on the 11am train from Penzance to London Paddington this afternoon.
"A delay of this magnitude is unacceptable and a full investigation will be carried out following today's train failure. The train suffered a fractured air pipe, which automatically applied the brakes as a safety measure.
"Our priority was to make sure our customers were looked after on the train and to get them moving as soon as we could. After extensive work to the train to get it moving, customers were met at Reading and London Paddington to make sure they could get to where they needed to go.
"We have already spoken to many customers but would urge everyone on the train to send us their details so that full refunds can be given in recognition of the poor service."
Many passengers took to Twitter to express their anger at the delay.
Jeremy Taylor wrote: "6 hours in a field on broken down First Great Western train today. Bad luck or incompetence?"
Judith Davey wrote: "Exeter - Paddington train broke down for 6 hours today - no water, toilets not working and poor info on journey status."
Mike Haydock tweeted: "Horrific journey with @FGW - train just started moving again after a five-and-a-half-hour standstill."