Melbourne's rail operator Metro says it is hoping to restore train services to five lines as soon as possible after workers accidentally cut through cables during works in the city's CBD.
Buses are replacing trains on the Craigeburn, Sunbury, Werribee and Williamstown lines, and other services are having to be redirected to Flinders Street to avoid the city loop.
V/Line says its services have also been prevented from leaving Southern Cross station.
Metro's Daniel Hoare says delays are expected across the network as they try to fix the problem.
"We've had a cable cut somewhere during works, somewhere in the city," he said.
"We've got five lines that are currently suspended, unfortunately we have to run buses in various sections of those lines, so we're asking our customers to look at our website for travel information."