Crossing the Don River, not once but twice. Tunnelling under dense downtown districts. Serving a growing number of transit riders using smaller trains.Those are some of the challenges the provincial government will have to navigate to make its proposed Ontario Line a success. Ed Levy, a veteran transit planner, said given the complexity of putting stations in dense areas such as Queen St. and Spadina Ave. he doubted the province’s timeline. (Andrew Francis Wallace / Toronto Star File)The light metro project is the highlight of the Progressive Conservatives’ blueprint for a $28.5-billion transit network spanning the GTA that Premier Doug Ford unveiled last week, and would replace the relief line subway proposal city council and the TTC have been pursuing for three years.The provincial government argues the 15-kilometre Ontario Line, which would run between Exhibition Place and the Ontario Science Centre via downtown and Leslieville, would achieve the 7.4-kilometre relief line’s goal of diverting passengers from the TTC’s overcrowded Line 1 (Yonge-University-Spadina), but could be built quicker and for less money because it would operate with smaller vehicles and would run above ground for part of the route.The government’s preliminary estimates say the line could be constructed for $10.9 billion and be complete as early as 2027. However, the project has yet to undergo the extensive planning and design work required to reach a firm budget and construction schedule, and key decisions about the type of trains and construction methods that would be used have yet to be made. Experts who spoke to the Star said the Ontario Line plan has some advantages over the city’s previous relief line plan, but could be more challenging and costly to deliver than the province has claimed.A map of the provincial government’s proposed Ontario Line, which will replaced the city’s planned relief line. (Government of Ontario)Bridging the DonExperts said one of the biggest advantages of the Ontario Line is that it would go as far north as Eglinton, where it would connect with the Ontario Science Centre station on the Crosstown LRT.By contrast, the $7.2-billion first phase of the council-approved relief line would only go as far north as Pape station on Line 2 (Bloor-Danforth), with a northern extension planned for a subsequent phase.Eric Miller, the director of the University of Toronto’s Transportation Research Institute, said pushing the route further north is critical to helping divert passengers off of Line 1.“If this line, whatever we call it, is to have serious impact on Yonge St., it’s got to go north of Danforth at least as far as Eglinton,” he said. The longer route means the Ontario Line would have to cross the Don River in two places: somewhere north of Pape Ave., and in the vicinity of Eastern Ave.Unlike the previous plan to tunnel about 40 metres under the Don, the Ontario Line could cross over the river, which Metrolinx, the provincial transit agency, says would be “significantly cheaper.”University of Toronto’s Eric Miller said one option for crossing the Don River would be to run trains across the existing 450-metre bridge at Millwood Rd. (Jack Lakey / Toronto Star File)Shoshanna Saxe, an assistant professor in U of T’s civil engineering department, said it’s “reasonable” to assume erecting bridges would be less expensive than tunnelling.However, for trains to safely cross, the bridges would have to be built on a shallow slope, which would require long run-ups on either side of the river and disruptive surface construction in areas where the tracks emerge from tunnels.“It will have more impact on the local community, but it will probably cost less money overall,” Saxe said.She noted while bridges would be cheaper to build, in the long term they would also require more costly maintenance than tunnelled track, which is protected from the elements.Near Eastern Ave., the Don is only about 50 metres wide, but building a bridge there could be complicated by the city’s plans to reconstruct the Gardiner Expressway.For the wider crossing in the Ontario Line’s northern section, Miller said one option worth investigating would be to run trains across the existing 450-metre bridge at Millwood Rd., which would be easier under the province’s proposal to use lighter vehicles.Tunnelling downtownThe Ontario Line would likely have to be underground through much of the dense lower part of its route between the Don River and the north end of Exhibition Place.The Ford government says the smaller trains required would make it easier to build, but experts say digging in the core will be expensive and complicated even if the tunnels are a few feet narrower than conventional subways.“Definitively saying it’s going to be a massive cost saving is overstating the point,” Saxe said.Utilities buried beneath the surface would have to be relocated, and the tunnels would have to be deep enough that construction and train operations wouldn’t disturb building foundations.The province proposes building downtown stops in the vicinity of King and Bathurst Sts., and along the Queen St. corridor at Spadina Ave., Sherbourne St., and Sumach St., as well as at interchanges at the existing Osgoode and Queen stations on Line 1. Ed Levy, a veteran transit planner, said underground stations at those locations would require at least two surface entrances each, and if the tunnels were built deep enough to avoid existing utilities and other structures, they would require “banks and banks of escalators and elevators” to carry passengers down to platforms.“It’s a very dense and very old part of the city, so this will be a very complicated undertaking no matter what kind of technology there is,” he said.The province has said it would accelerate construction by using its authority to expedite regulatory approvals and utility relocation, and by procuring the project through a public-private partnership that would transfer schedule risks to the private sector.But given the logistical complexity involved, Levy said he doubted the province’s timelines are achievable.“If they think they’re going to get all of this planning and analysis done plus building by 2027, they really must be on cannabis or something. I can’t imagine it,” he said.GO corridor The Ontario Line’s western terminus near the Exhibition GO station would introduce the new challenge of spanning the corridor where GO’s Barrie, Kitchener, Milton and UP Express lines converge as they approach Union Station. Building a bridge over the busy corridor would likely require temporary suspensions of GO service, and Saxe said that given the surrounding density it would likely be easiest to tunnel beneath the tracks. Some critics have raised concerns about the Ontario Line disturbing Fort York, the national historic site about 800 metres east of Exhibition GO. (Justin Greaves / Metroland File)Some critics have raised concerns about the line disturbing Fort York, the national historic site about 800 metres east of Exhibition GO. But Saxe said the experience of other cities shows heritage properties can be protected. “There’s a subway station right up under Westminster in London. You can build under really sensitive places and take care of them, you just have to be careful,” she said. Building capacityCritical to the Ontario Line’s success is the idea that while its trains would be smaller, they could provide “similar peak capacity” to a full-sized subway. Metrolinx says it could achieve this by deploying driverless trains that could run closer together, enabling service of between 36 to 40 trains per hour. On good days, the TTC is currently able to run 28 trains an hour on Line 1 through Bloor-Yonge station. Experts are skeptical about the province’s plan. Saxe pointed out the benefits provided by running automated trains close together could also be realized with full-sized vehicles, which could carry more passengers.Levy argued that more than how fast trains operate or how close they run together, a greater factor determining capacity is how long it takes to load and unload passengers in a station, something smaller trains wouldn’t necessarily help with during peak crowding times.He also questioned the wisdom of building a system for smaller trains through the heart of downtown, where transit demand is only expected to grow in coming decades.“If anything in their entire map is deserving of something that offers subway-level capacity, it’s that line,” he said.A new yardThe TTC intended to use Line 2’s existing Greenwood Yard to store the additional trains required to operate the relief line. But because the Ontario Line would use smaller trains that wouldn’t fit on the TTC’s existing tracks, a new yard would have to be constructed. Metrolinx officials have said they believe there is space for a new yard for the Ontario Line’s lighter, smaller vehicles somewhere in the Thorncliffe Park area. (Lucas Oleniuk / Toronto Star File)Metrolinx officials have said they believe there is space for a new yard somewhere in the Thorncliffe Park area. The full cost isn’t known but the province says it would be within the $10.9 billion project budget.Ben Spurr is a Toronto-based reporter covering transportation. Reach him by email at email@example.com or follow him on Twitter: @BenSpurrTOP STORIES, DELIVERED TO YOUR INBOX.