Passengers had to be evacuated from a train that broke down at the Perth Underground station this morning.

The south-bound train lost power shortly before 8:30 am, leaving passengers unable to get off or on.

Another train was brought in to shunt the stalled train off the line.

Passengers tweeted they were being evacuated off the train into a dark tunnel.

Former Labor MP Tony O'Gorman was among passengers on a following train that was used to try to shunt the broken down carriages from the tunnel.

"So they moved up to the back of the other train, and you could feel it just touch very gently," he said.

"And they were trying to push but obviously it wasn't working."

Mr O'Gorman says a short time later they were instructed to leave the train.

"We were evacuated through a tunnel onto a platform at Perth Underground," he said.

"And then of course the two escalators weren't working. One of them has been down for a few weeks. So everybody's clamouring to get up the stairs.

"It's peak time so it was pretty packed with passengers. Not as packed as I've seen them, but every seat was taken and there was quite a number of people standing.

"Because everybody had mobile phones they were texting people. Ringing their bosses, ringing their workplaces saying 'I'm going to be late, I'm stuck on a train, I have no idea how long I am going to be'.

"So everybody was pretty calm. Pretty collected. Very well behaved. All filed off in single file and up through the tunnel, onto the platform and out.

"There was a platform at door level in the tunnel. Obviously a service platform."

A Transperth spokeswoman says delays are expected to be minimal.