A serious shortage of Toronto Transit Commission operators nearly derailed plans for extended subway service for the thousands enjoying this weekends Nuit Blanche, Toronto’s all night art party.

Forty operators were needed to work overtime to run the 20 trains running along Line 1 and Line 2, but only 10 signed up for the voluntary overtime, meaning 30 supervisors had to take the controls.

Many of the drivers refused to pick up the shifts because of what the TTC’s union told CityNews is a “grass roots movement.”

TTC’s move to single operated subway trains on the Sheppard line is one of many concerns subway operators have. The Amalgamated Transit Union’s head Bob Kinnear tells Citynews “this could definitely be the beginning.”

TTC spokesperson Brad Ross wouldn’t comment on why staff declined to pick up the voluntarily shifts but said “we knew the union was thinking of not having operators available for that evening.”

Ross said safety for passengers was of paramount concern this weekend.

“Platforms can get very crowded, very quickly and anything more than ten minutes [wait for passengers] and it would have been unsafe for customers,” said Ross. “We had supervisors on standby in case operators chose not to work that overtime.”

For 2016, the TTC has 40-million budgeted for overtime, according to Ross.

Kinnear said in recent years operators have been expected to work 60 to 70 hours and that “the TTC absolutely depends on this overtime.”

Though the TTC doesn’t see it that way, stating that occasional overtime “saves the TTC money in employee and benefits costs and it ensures we are able to meet service due to unforeseen circumstances that can arise.”

Kinnear believes more overtime refusal by TTC operators is coming down the tracks.

“There’s some organization going on,” he said. “We will support our employees, if they decide they don’t want to work overtime.”