The $80 million back-up system for Melbourne's rail network may have triggered the glitch that brought down the city's train system on Thursday, stranding or delaying an estimated 175,000 passengers.

Housed in the heritage-listed former cable tram engine house on Nicholson Street in Fitzroy, Metro's state-of-the-art Disaster Recovery Site contains hundreds of servers in a secure environment with uninterruptible power and is the last line of defence for Melbourne's rail commuters.

A packed train at Caulfield heads to Dandenong after the outage. Credit:Joe Armao

But it failed at the precise moment it was needed on Thursday, plunging the city into chaos on the cusp of the evening peak.

The commuter chaos was exacerbated by a trespasser who entered one of the City Loop tunnels at Melbourne Central station soon after trains had started to run again.