Electrification of the GO rail network may not be done within 10 years, according to the regional transit agency Metrolinx, raising new questions about the John Tory "surface subway" promise that relies heavily on the province's plan.

Metrolinx – which had previously called the decade-long timeline to change GO into an electrified and higher-frequency service "aggressive" – released Tuesday a great deal of new details about its plans. The report, which is part of the agenda for a board meeting later this week, offered new insight into the scale and complexity of the project.

Bruce McCuaig, chief executive officer of Metrolinx, said in an interview that they are still working out how much they can do within the next decade.

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"Does every little part of the system see electrified service in that 10-year period?" he said. "We're still too early in the planning to figure out, you know, what actually will be the priorities for completion in the 10-year horizon."

New detail for the ambitious project is laid out in the 30-page presentation to be made to the Metrolinx board. Moving to a high-frequency service will be a major shift, requiring everything from different rolling stock to laying about 340 kilometres of new track. Stations will be renovated and new ones built. The model that now has most GO passengers driving to the station and leaving their vehicle there will have to be rethought.

If the challenges are substantial, the promise is equally large. Running trains more frequently will turn the GO rail network into a service somewhere between straight-up commuter rail and something closer to a subway. Although ridership studies are still being done, the transit agency has found that additional trains put on the existing service fill up quickly.

"There's going to be a transformed and different kind of GO Transit at the end of the 10 years," said Mr. McCuaig, who noted that improvements will roll out over the decade and that service will get better even before full electrification.

Although the 10-year window that has been touted by politicians since GO electrification was promised in the spring budget, the Metrolinx CEO said it was premature to say the goal was being walked back.

"Our objective is to try and deliver as much as we possibly can in the 10-year horizon," he said. "I think what we need to come back with is, what is that program going to look like and what parts of the program get done … and, after 10 years, what is the outcome?"

Timelines matter at both the provincial and municipal levels.

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Premier Kathleen Wynne has pledged to invest $15-billion on infrastructure in the Greater Toronto and Hamilton area over a decade, and residents will expect to see progress by the next election. And, with transit and congestion shaping up as the key issue of Toronto's mayoral race, Mr. Tory has staked his campaign on a plan that requires the province to come through.

Mr. Tory has promised to provide what he calls SmartTrack – which assumes GO electrification of parts of the Kitchener and Stoufville rail lines and adds short stretches of new rail – within seven years.

"I am confident in my plan to build SmartTrack in seven years," he said in an e-mailed statement. "Metrolinx is referring to the whole network. SmartTrack is two lines out of a network of seven and a spur to Mississauga."

In the interview, Mr. McCuaig reiterated the Metrolinx position that it is too early to say which lines would be electrified first. He said that work could be done on more than one line at once and that the opinions of the affected municipalities will be one factor in the sequencing decisions.

Among the unknowns of the network-wide project is how long it will take to electrify Union Station, which must predate any individual rail corridor moving to faster service. The construction work on that alone will take three to four years, Mr. McCuaig said, but it's unclear how soon that might start.

"I'm not going to give you a specific time because then you're going to hold me to that time," he said. "Union-Pearson, first of all, we don't have all the environmental assessment approvals yet, we need to get those in place. You know, the power supply to the corridor is a pretty important thing, we don't have that approval yet. And we need to work with the province to get all the treasury board approvals over the coming months for the program."