TTC brass were blaming a small fire and a "smouldering" cable for a three-hour service delay on the Bloor-Danforth subway line Wednesday that turned the morning commute into a nightmare for thousands of riders.

Investigators have yet to determine the cause of the fire underneath Yonge station or the damage to the cable that required 18 metres to be replaced.

TTC spokesman Brad Ross, however, tweeted a photo of a damaged piece of wood that covers the third rail along the track.

"The culprit," Ross tweeted. "The burned wood is a cover board over the 3rd rail, which gives trains power. Cable being replaced."

The culprit. The burned wood is a cover board over the 3rd rail, which gives trains power. Cable being replaced. <a href="https://t.co/JswMxOMeUJ">pic.twitter.com/JswMxOMeUJ</a> —@bradTTC

To deal with both safely, service had to be halted between St. George and Pape stations, TTC CEO Andy Byford explained.

"I hate the fact that customers are having to go back on shuttle buses this morning," Byford told reporters at the scene, acknowledging passengers' frustration.

Passengers felt the same way.

"I don't think there are enough shuttle buses running," one woman waiting at Pape station told CBC. "Everybody is going to be late for work. This is crazy."

Passenger Joseph Pasher said there weren't enough people directing passengers on how to get to onto shuttle buses. He also said problems with the track are far too common.

"Every day, either going to work or coming home from work, there's always something that says fire," Pasher told CBC. "Whatever's causing these fires, I mean, it's every day. Find out what the problem is with the fires and fix it."

A cover that protects the third rail underneath Yonge station is seen with extensive damage on Wednesday morning. (Brad Ross/TTC)

Even though passengers may criticize the TTC for suspending service, Byford said he won't compromise safety.

"I will always instruct that we suspend the service and get the fire service in rather than put customers' safety at risk," he said.

While firefighters put the blaze out quickly, Byford said that replacing the cables was the more time-consuming part of the process. They are large and heavy, and had to be dragged through the station and down to the tunnel, he said.

Then, work cars had to be cleared so a test car could be run through the tunnel to ensure that the repairs were done properly — and to confirm that signals and switches weren't damaged.

The TTC moves 1.8 million customers each day, and "most journeys are completely without incident," Byford said. "Obviously we'll get to the bottom of this. We need to understand what it was that caused the cable to smoke."

Full service did not resume until 9:15 a.m.