Robin Smith, 35, found herself stuck in the toilet on a CrossCountry rail service from Southampton to Manchester Piccadilly after the electronic doors failed to open.
The CrossCountry route was much loved by train fans and ran in one direction southbound, leaving Aberdeen at 8.20am before arriving in Penzance more than 13 hours later at 9.31pm.
CrossCountry was given the lowest rating of 77 out of 100 in the Transport Focus report after suffering disruption for years, partly because of a driver training backlog and industrial action.
Daisy Mumba, 32, who was travelling from Slough to Sheffield, was forced to sit on the floor next to the toilet of a CrossCountry train service because she could not get to the seat she had booked.
CrossCountry runs the 35-stop service from Aberdeen to Penzance which takes more than 13 hours and covers 785 miles - but it will no longer run from May due to a timetable shake-up.