Two inbound trains ground to halt in the Melbourne loop about 8.30am, due to an overhead power issue.

Both trains lost power, stranding commuters underground for 50 minutes.

"Only one train was faulty, but both were affected because of the overhead power issue," Metro Trains spokeswoman Larisa Tait said.

Three other trains in the city loop at the time were stopped at platforms, allowing the passengers to disembark.

"But passengers on the two trains that were not a platforms were stranded," Ms Tait said.

One of the two affected trains was able to move on to Melbourne Central station after 50 minutes, while the other progressed to Flagstaff station, where passengers disembarked.

Commuters on the Pakenham, Cranbourne and Frankston lines were warned of 15-minute delays as other trains were diverted directly on to Flinders Street station.

Normal services through the Caulfield group tunnel on the city loop resumed shortly before 10am.