One X user wrote: ''I have a ticket but can’t download it - any advice on what to do? Will the ticket be honoured even though I obviously can’t get through the barrier at the station?''
A second fumed: ''How many more times this month?!''
Meanwhile, a third added: ''Literally a joke at this point.''
This is the third app outage the company has faced this month.
Responding to complaints, Trainline's X account posted: "We’re experiencing technical difficulties with our app and website which means some customers may be unable to search for or book rail and coach journeys.
"We’re very sorry for any inconvenience this may cause however please be assured we’re working hard to fix it."
Trainline has suffered issues with its app and website this morning as Brits head to work.
Over 1,500 reports have been made via DownDetector, with many users unable to buy tickets for trains or coaches.