For anyone following discussions of transit in Toronto, the debate on how to provide transit to Scarborough has been front and centre. Scarborough consists largely of older moderately dense post-WWII suburban neighbourhoods, and has become home to a large portion of Toronto's lower income population in recent decades and has relatively high transit mode share (20-40% in most neighbourhoods). However, it remains relatively disconnected by transit from central Toronto.
Early plans for Scarborough included two subways with Scarborough Town Centre being a major focus. Scarborough Town Centre was intended to be the downtown of Scarborough, although it has not developed to quite the level initially anticipated. It now consists of a shopping mall surrounded by a few condo towers, office buildings and civic buildings.
The transit plan was to have one subway line running along Sheppard Ave E connecting Scarborough Town Centre to Downsview, and a rapid transit using somewhat different technology from the rest of the subway system connecting Scarborough Town Centre to the Bloor-Danforth Subway at Kennedy Station and to Malvern called the Scarborough RT.
The Sheppard Subway was only partially built in 2002, the section from Yonge & Sheppard to University-Spadina subway at Downsview Station, and the section through Scarborough from Don Mills to Scarborough Town Centre unbuilt. The Scarborough RT only built up to Scarborough Town Centre rather than Malvern, and is now in need of replacement for various reasons.
As a result, there have recently been many different proposals for the construction of rapid transit line extensions and new transit lines to Scarborough. These include extending the Bloor-Danforth Line along the Scarborough RT right of way or perhaps a slightly different route, and extending the Sheppard East subway into Scarborough. One alternative has been do build these as light rail lines, along with additional light rail lines along Markham Road, Kingston Road and Morningside Avenue. Finally there are plans to improve service along the commuter rail lines of Toronto, two of which pass through Scarborough, by increasing frequencies, operating hours (one rail line is rush-hour only) and electrification which could allow for either faster travel or additional stations. This would allow these rail lines to function much more like rapid transit and boost their usefulness to Scarborough residents.
One major criticism of the subway plan is that the low densities will result in ridership levels too low to justify the high cost of subways, with the existing Sheppard East subway, and sometimes Scarborough RT cited as examples or rapid transit in similar moderate density suburbs with low ridership. Another related criticism is that due to the wider stop spacing (and shorter length due to higher costs) the subway will serve fewer people than LRT due to fewer people within walking distance of stations.
However, here is a map showing the commuter shed of some of the relevant currently existing transit lines. This is based off the fastest transit route that google recommends for getting from various parts of the city to Yonge and Bloor (more or less downtown). Red is the commuter shed of the Sheppard East subway, light blue is the portion of the commuter shed of the Bloor-Danforth subway that's within Scarborough, and black is the commuter shed for the Scarborough RT.
So clearly for most people in Scarborough, the Bloor-Danforth Subway is the best option. For most people in Scarborough, this doesn't involve walking to the subway, but rather getting on a bus and then transferring onto the subway. Note how small Sheppard East's commuter shed is by comparison, even including the portions within North York. This suggests that density plays only a relatively small role in Sheppard East's low ridership, and that how it fits into the overall transit network is bigger problem.
How so? The commuter shed is largely restricted by competing bus routes as Sheppard East functions merely as a feeder line for the Yonge Subway. For areas near Finch Ave E or Steeles Ave E, it's faster to take the bus along these roads and transfer onto the Yonge Subway at Finch Station than to take the bus and transfer onto the Sheppard Subway and then transfer again onto the Yonge Subway. The 199 Finch Rocket seems to be especially competitive, being an express route. South of the 401, it is again faster to take various bus routes to the Yonge Subway directly rather than the Sheppard Subway, and in most of Scarborough, it's faster to take various buses down to Kennedy, Warden or Victoria Park stations on the Bloor-Danforth Line. Further out towards the suburban outskirts in York Region, Durham Region of the furthest reaches of Scarborough, GO commuter buses and trains tend to be more competitive.
The problem is that the Sheppard Subway is too short to make sense as a subway line, especially being in a relatively low density setting far from downtown with relatively few major destinations along the Sheppard Subway, especially when it comes to major destinations that are difficult to get to by car. While subways are generally faster than buses, they partly achieve this by making fewer stops. Since North York already has the Yonge Subway, the speed advantage is the Sheppard Subway is only realized over a short distance, limiting the amount of time saved over buses.
For example, if you want to from Finch and Leslie to Yonge and Sheppard, you can either travel 4 km east at bus speeds and 2 km south at subway speeds (Yonge Subway), or 2 km south at bus speeds and 4 km east at subway speeds (Sheppard Subway). So the difference is 2km at bus speeds vs 2km at subway speeds. However, taking the Sheppard Subway will involve walking more to reach transit and an additional transfer. Or if you happen to live really close to the arterial intersections, walking distance might be similarly short, but you can take the 199 Rocket which is almost as fast as the subway but requires one less transfer.
However, these arguments don't necessarily apply to a Bloor-Danforth extension further into Scarborough since there are currently no major competing rapid transit routes anywhere close by. As a result, if none of the other transit proposals for Scarborough are built, the existing commuter shed within Scarborough will be better served. The areas from Eglinton southwards in SW Scarborough would feed into the existing portions of the Bloor-Danforth subway and so quality of service would be similar. However, central Scarborough around Lawrence, Ellesmere and Sheppard would be better served. A Scarborough subway extension could even "poach" riders from North Scarborough who are currently taking buses along Finch and Steeles to the overcrowded Yonge Subway.
When you include transit riders taking the bus to subway stations rather than just walking, a Scarborough subway would impact the entire area of Scarborough, a much larger population than Sheppard East's commuter shed, at about 600,000 people vs less than 100,000.
Will this be enough of a difference to justify a subway? Hard to say, people would probably be more likely to take the subway or other rapid transit if it's within walking distance rather than having to take a feeder bus. Still, feeder buses are certainly important. The existing subway stations in Scarborough like Warden and Kennedy which have feeder bus routes get several times higher ridership than many stations in central Toronto that are higher density but lack major feeder routes. It also doesn't mean that the Bloor-Danforth Extension should be made unnecessarily expensive just to be able to say it's a subway. If the existing Scarborough RT infrastructure can be used in a way that will significantly reduce costs over a tunnel-bored subway along McCowan, I think it should be. As long as the route is grade-separated, it doesn't matter that much if it's underground or not.
There is a big if though, which is "if no-other significant competing rapid transit routes are built". So if the Ontario government remains committed to upgrading the commuter rail network, or funding can be gotten for multiple Scarborough light rail lines, a Scarborough Subway may not be necessary. It's important to have good communication between Toronto and the provincial and federal governments.
At the same time, if the Scarborough Subway does turn out to be unneeded, it's also important to ask if a light rail line along the Scarborough RT corridor makes sense, and whether multiple alternative lines could do the job better. If there are several competing lines, a transfer required at Kennedy Station, and short stop spacing, usage might be relatively low. This is especially true when considering a good chunk of the Scarborough RT goes through low density industrial areas with restrictions to redevelopment into high density TOD offices and housing, whereas most of the Sheppard East Subway corridor is seeing a great deal of intensification.