Subways were up and running on Sunday morning after contractors working at Toronto's Union Station hit a water main and forced the TTC to halt subway service on the Yonge-University line between Bloor and Osgoode stations.

The burst main spewed water into a subway tunnel and flooded the tracks, prompting the TTC to use shuttle buses to ferry passengers around the disruption.

TTC chief operating officer Andy Byford said the damage is "extensive."

"It's clear that there is a lot of equipment absolutely soaking wet," Byford said. "This is very sensitive safety equipment and signalling equipment; it's not supposed to get wet."

Firefighters worked for hours pumping water out of the tunnel.

Byford wouldn't estimate how much the cleanup and repairs might cost, but it's expected to be high.

"There will be costs, obviously." he said. "The cost of the shuttles, the cost of the repairs.

"Right now that's not my priority. It's getting people moving, minimizing inconvenience and getting it all repaired."

Byford said he plans to sit down with EllisDon, the contractor working on the Union Station renovations, as soon as possible to determine exactly what happened and who should pay for it.