Melbourne commuters have been forced to rush to the back of a train after power line connectors exploded during the morning peak hour.

Witnesses reported at least six explosions from the roof of an outbound train, which came to a halt between Flinders Street and Richmond station about 7.20 am on Thursday.

"I was just pulling my car into the Fed Square car park about 7.15 this morning ... I heard this massive sort of spark and crack and saw some smoke coming up," Ben Yeo told 3AW.

Mr Yeo, who recorded a video of the incident, said he saw a small fire coming from the roof of the train.

Another witness told AAP they saw six, bright flashes and what appeared to be flame and smoke coming from the roof the train, although a Metro Trains spokeswoman insists there was no fire.

"At no point were passengers in danger but we understand that it would have been concerning for our passengers to see sparks," Metro spokeswoman Sammie Black said.

"As a safety precaution, the Metropolitan Fire Brigade attended and left shortly after as there was no safety risk or fire."

The MFB attended but quickly left, describing the incident as a "non-event".

The loud explosion sound sent 30 passengers on the outbound train scrambling to the back carriage where they were stranded for up to an hour.

Another 300 people were riding an inbound train subsequently brought to a standstill.

Metro staff arrived just before 8.30 am to help

Commuters disembarked both trains about 8.30am with the help of Metro staff and walked along the tracks to Flinders Street Station.

Delays were expected into Thursday afternoon while repairs and investigations were conducted.