Sydney commuters have experienced major delays on Tuesday morning after a maintenance vehicle derailed between Circular Quay and Wynyard stations, but have been promised a smooth-running service by the afternoon.

A maintenance vehicle became jammed on the city loop after 4am on Tuesday morning, with the crew unable to pull it free, Transport for NSW chief operating officer Howard Collins said.

“Normally you can get out of anything, it’s a bit like that ute you see on TV, but sometimes you get beached and this thing had wedged itself into the rails," Mr Collins said.

“Their pride has probably been pretty hurt.”