There is going to be more sprawl,what ever you do. It just depends how you shape it.
The GTA is going to add 2 million people in the next 20 years and the area around it another 1 million. Looking at the Urban growth boundary it won't take long for Toronto to reach it't limit. People will simply move to the other side of the green belt.
Only so many people will want to live in a condo most will still want a house, plus for many it will be the trade off of moving out to cheaper land and property and the commute back.
If you don't develop a rail based system the freeways will extend in a loose grid to serve the spread of one continuous ring around the city. You could instead have discrete settlements clustering around railway stations, which if you also cluster retail and commerce near those stations, then you allow commuting between settlements to be done by rail. As long as you develop a mix of long distance express lines and local services.
Older settlements that were too far from Toronto to grow, could now from the nuclei of expanded suburbs, rather than endless subdivisions. As the population rises more employers will locate closer to these workers. Depending on land use patterns you could have a lot more local commuting by rail . A lot of short distance travel would still be by car, but with faster rail alternatives many would prefer using rail for long trips as to have a consistent journey free of congestion.
In the UK many employers are now abandoning out of town business parks for city centres, where they find it easier to recruit staff who have access to good public transport and have the use of good shops, restaurants and services they can use in lunch and after work without having to drive somewhere else. The larger business parks are now trying to develop residential elements so they have the population base to at least provide some local shops and services. That's not to say the business park is dead yet, but many smaller ones are struggling and no longer attract the top companies.
With some work the South West of Ontario could become a network of medium sized cities with people commuting in all directions. Local rail could run from London and Kitchener towards Hamilton and Niagra for example. The area could develop it own strong regional economy, while still maintaining strong links to Toronto.
It's an expensive vision, but it would mostly use existing railroads and would uses much smaller trains than those bound for Toronto. But a stronger Non Toronto economy where maybe fewer people felt the need to move to Toronto would surely have tremendous political appeal.
