The head of Toronto's transit agency is leaving after a five-year tenure that will be capped with the opening of a new subway extension.

Andy Byford announced Tuesday that he would be heading to New York to run that city's buses and subways. He will stay at the helm of the Toronto Transit Commission only long enough to open the subway connection to Vaughan, scheduled for the middle of next month, explaining that the short notice of his departure was because of the lengthy process of sorting out a U.S. visa.

His exit comes after a tumultuous time that solidified the status of transit as the hottest political issue in the city. He led the agency to a major industry award – earlier this year, the TTC was named transit system of the year by the American Public Transportation Association – but his tenure was also a period in which fares rose repeatedly and ridership began to flatline.

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Mr. Byford was elevated to the top job at the TTC unexpectedly quickly. He had been chief operating officer and was being groomed to lead, but had to take over earlier than expected after his predecessor was fired for running afoul of his political bosses. Five years later, Mr. Byford is leaving to praise from both politicians and other transit officials.

"What I am most proud of … is changing the prevailing culture at the TTC," he told reporters Tuesday, giving thanks to the 15,000 employees of the agency. "I believe that I leave behind a management team that is on par with any transit agency."

Mr. Byford will begin in January as president and CEO of New York City Transit, a move that comes amid dwindling confidence there in that city's subway system. A recent investigation by The New York Times stated that "the needs of the aging, overburdened system have grown while city and state politicians have consistently steered money away from addressing them." The result has been a system where on-time performance is plunging and ridership has declined.

Mr. Byford acknowledged that he is taking "arguably the toughest job in transit right now."

TTC chair Josh Colle said he would ask the board later this month to affirm deputy CEO Rick Leary as the acting head of the TTC. A permanent candidate will be sought and is expected to be in place by the middle of next year.

The timing of Mr. Byford's replacement means that the agency could be in transition when the thorny issue of the Scarborough subway extension returns to political prominence. The controversial project is expected to come back to council once 30 per cent of the design work is done, some time next year, a milestone that could be accompanied by a higher price-tag and another round of fighting over whether this is the best use of transit dollars.

Blogger and transit watcher Steve Munro said that having an interim CEO during this process could limit the agency's ability to offer advice.

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"The difference it makes is that the position the TTC takes will be dominated by the position that the board, who are largely members of council, take," he said. "Rick Leary's a nice guy, but I don't expect him to stand up in front of council and tell [subway advocate] Glenn De Baeremaeker and the Scarborough caucus that their Scarborough subway is an increasingly expensive waste of money."

As TTC chief, Mr. Byford was a classic railwayman, an operations guy content to devolve transit planning to another city agency. He obsessed over running the system as efficiently as possible, focusing much of his attention on modernizing the agency and insisting on a greater focus on customer service. But he also showed the political touch needed to operate in a municipal environment in which transit is hotly contested, and plans can be upended with each mayoral election.

He predicted Tuesday, though, that such a policy upheaval is unlikely to happen again in the near future. "Politicians know that the people of Toronto want and need and deserve better transit, so I don't think we'll see a big sea-change," he told reporters.

A native of Britain who also has worked in Australia, Mr. Byford brought in international talent to the upper ranks of the TTC, an agency that in the past has been more likely to promote from within. He also made a point of diversity and noted with pride that half of the executive ranks, which had been all-male only a few years ago, are now women.