Commuters were bumped off their Chiltern Railways train at Rickmansworth this morning due to a signalling fault.
A signal failure near Amersham at 8.20am caused problems for the Aylesbury and Marylebone service.
The rush-hour disruption led to commuters being told to get off the train at Rickmansworth and use the London Underground service.
Engineers began fixing the problem at 9.05am.
The line has now reopened, with some knock-on delays of up to 20 minutes.