A campaign to retain guards on trains has been reinforced by the "total chaos" to services which left passengers stranded in the dark on a broken-down train for more than an hour, a union said.
The Thameslink train broke down in north London on Thursday and a recovery train also broke down. Replacement buses did not arrive until 10pm, hours after the train first broke down.
Passengers vented their anger on social media, one tweeting: "Sitting on a broken down train in the dark for over an hour."
Thameslink is owned by Govia Thameslink Railway which also runs Southern, the operator at the centre of a bitter dispute over the role of conductors.
Southern services were also disrupted on Thursday because of separate problems, with delays reported on both operators again on Friday.
Mick Cash, general secretary of the Rail, Maritime and Transport union said: "Not only does the total chaos on GTR last night reinforce RMT's campaign to retain the guards on Southern, it also forces the issue about bringing guards back on to the Thameslink services back up the agenda.
"We have to end the madness of leaving the driver as the sole person ?to cope in these emergency situations. This lethal gamble with rail safety has to stop before we have a tragedy on our hands.?
"How much more of this unprecedented chaos on Govia Thameslink Railways have passengers got to endure before the Government pulls the plug on this basket case franchise?
"They are a sick joke and are getting worse by the day. Transport Secretary Chris Grayling and the Tory Government should be hanging their heads in shame and instead of propping these racketeers up they should be slinging them off the tracks and taking the services back under public control."
The broken-down train was a driver-only operated service, said the union.
A Thameslink spokesman said: "We want to sincerely apologise to passengers who were delayed last night on our Thameslink services due to a broken-down train blocking the line.
"Lines were closed for almost three hours and we ran a reduced service overnight to get trains back in place for this morning.
"Any passenger who was delayed for more than 30 minutes can claim delay repay. Details can be found on our website."