This week, candidates for mayor were debating how best to support the arts when Olivia Chow pulled a little stunt. She brandished a hand-drawn map of her rival John Tory's SmartTrack transit plan, written on the back of an actual napkin, and called it art.

Silly? Yes. This bit of mischief was a way to get around the moderator, who was trying to keep the candidates on topic. Desperate? You might say so. Mr. Tory is the front-runner and, with less than four weeks to go, Ms. Chow is doing all she can to draw attention to the flaws in the main plank of his platform.

All the same, that scrawled map deserves a second look. The questions she has about SmartTrack aren't trivial.

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Mr. Tory's $8-billion plan would use GO Transit track to create a 22-stop "surface subway" line spanning the city. But part of the line would depart from the existing GO route to head west along Eglinton Avenue from Weston Road.

Two problems. First, how do you get those trains to make the sharp left from the GO track onto the Eglinton corridor? Ms. Chow points out that there is a whole neighbourhood, Mount Dennis, around the Weston-Eglinton intersection. Would some of it need to be demolished to make way for new track? If you tunnelled under it instead, what would it cost?

Second, where do you put the line as it continues along Eglinton? It is supposed to go on a right-of-way once reserved for a highway, the Richview Express- way, that was never built. The trouble is that Build Toronto, the city's real estate agency, has been selling off strips of the right-of-way. On one strip, townhouses are already under construction. What to do? As Ms. Chow puts it, trains do not have wings.

Mr. Tory now concedes that making the Eglinton spur work may require some tunnelling. Again, how much and what will it cost? Would the trains dive under one patch of housing on the right-of-way, rise to the surface again, then dive under the next patch? This is supposed to be a commuter train, not a roller coaster.

Mr. Tory brushes aside such questions as naysaying and negativity. "There are people who will come up with every reason why we shouldn't do this, or we can't do it or there's obstacles that stand in our way or so on," he said at a visit to The Globe and Mail editorial board on Wednesday.

When Toronto built the Yonge subway, he says, engineers found a section of solid rock in their way. "And did they give up and just fill in the hole and say we'll just have people walk to work? Of course they didn't."

The difference is that the engineers in his story didn't know about the rock before they started digging. Anyone with a search engine and a few minutes time can find out that stretches of the right-of-way are being developed and so not available for a heavy-rail transit line.

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Mr. Tory says his campaign knew about the right-of-way issues, but proceeded regardless, confident that "the marvels of engineering in 2014" can overcome any hurdles. Maybe they can. But it is perfectly legitimate for Ms. Chow to ask how and at what price.

It is equally fair for her to ask in debates whether he can assure residents that building the western spur of SmartTrack would not require the demolition of local homes. When asked at the editorial board whether he can give such an assurance, he said: "Well, the bottom-line answer is that, where you have to go below or around obstacles, that's what you'll do. So the answer to the question is yes."

The questions Mr. Tory is being asked about SmartTrack are more than just the quibbles of weak-kneed skeptics. The most serious of those questions involve Mr. Tory's funding proposal, which would employ something called tax-increment financing to raise the city's share of the bill. The issues with the Eglinton section suggest the potential problems with this plan, pushed out in the heat of an election campaign, go beyond that. Transit analyst and blogger Steve Munro concludes in a recent article that "SmartTrack cannot be built as proposed."

Sure it was a stunt, but when Ms. Chow drew her map on the back of a napkin, she was making a point that needs to be made.