Trains are returning to normal ahead of the afternoon rush, after urgent signal repairs disrupted train services to and from Western Sydney this morning.

Transport for NSW has urged travellers on multiple lines to allow for extra time for their journey following the "complicated" issue at Harris Park, which has since been rectified.

Rail operations executive director Jas Tumber said the signal failure started at 4am and was resolved about four hours later.

"All those lines are fully operational, and the teams are working very hard now to recover the network and get the timetable back in order, which will be by around midday," he told ABC Radio Sydney. 

"The biggest impact for us at the moment is between Blacktown and Penrith where we do have some additional service gaps."

Transport for NSW said passengers should allow plenty of extra travel time on the T1 North Shore & Western Line, T2 Inner West & Leppington Line, T5 Cumberland Line, as well as the Blue Mountains Line. 

Trains that normally run to Parramatta from the City on the T2 line are no longer terminating early at Granville.

Services on the T5 line were temporarily suspended but are now running at a reduced frequency.

"Trips may take longer than usual, with some gaps in service expected," Transport for NSW said in a statement.

"Passengers should also listen to announcements and check information displays for service updates."

Some trains were being expected to run more than an hour late.

A commuter told ABC News the delay was a "very inconvenient" start to the week. 

He had been waiting for at least 1.5 hours at Harris Park, on top of his usual commute from Lakemba to Parramatta.

"Starting your week on this note will be pretty frustrating ... let's hope for the best," he said.