Angry commuters attempted to "kick down gates" at St Albans station after a major signal failure caused fresh delays on Thameslink.
Network Rail has apologised for the service on Thursday morning after a six-mile stretch of signalling failed at Luton.
It meant that no trains could run south of Bedford.
A spokesman for Thameslink said no gates were "kicked down" at the station, but a few passengers tried to push gates until staff intervened.
Network Rail said the problem was fixed by 7.30am but disruption continued until around 4pm.
At St Albans City station witnesses reported an "absolute fiasco" after the trains were cancelled.
Peter Saunders said hundreds of angry passengers were left outside barriers due to "crammed platforms" after the disruption.
Dr Saunders tweeted: "Absolute fiasco here at Thameslink Rail St Albans. Trains cancelled. Platforms cancelled. Barriers closed."
He added: "Angry people kicking down gates and pushing through."
Another commuter, Conor Quinlan, tweeted: "@TLRailUK - your shambolic service continues - this was the view outside St Albans station at 8.13am. Trains cancelled with no notice, no communication to your staff leaving them clueless. Hopeless!"
A spokesman for Thameslink said: "For passenger safety we closed the gates for short periods to manage the numbers of people on the platform. Passenger announcements explained this."
Network Rail, who maintain the track, signalling and power supplies, said: “We would like to apologise to passengers experiencing delays this morning owing to a signal failure at Luton.
Our engineers have now fixed this issue and we are working with train operating companies to recover normal service as soon as possible.
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“However, there are still residual delays between Bedford and London St Pancras, so passengers are advised to check their journeys before travelling throughout this morning."
