I lived in Toronto for a while, and was a subway commuter:<p>- Built really early. There are a lot of cities that are <i>just</i> getting into the mass transit game, Toronto has been in it for 50 years, and this has allowed time for the culture to adjust, and for large housing complexes and shopping centres to develop around subway stations.<p>- Expansive, a lot of American cities start with small mass transit systems that don't run very far away from downtown. Toronto's advantage is that the two extreme ends of the subway line are <i>very, very</i> far from the downtown core, so subway commuting is a <i>realistic</i> option for people looking to live in cheaper neighbourhoods.<p>- Not reserved for poor neighbourhoods. In fact, the poorest areas of Toronto are entirely <i>unserved</i> by the subway system - the subway covers large swathes of lower middle class, middle, and upper-middle, and even upper-class neighbourhoods. This forced demographic has made the system in general much more welcoming as an option for people who would otherwise have the freedom to buy cars.<p>- Extreme density downtown. Once you leave downtown the distance between stations grows very dramatically, but in downtown there really <i>is</i> a station every 2-3 blocks, and you are <i>never</i> more than 5 minutes away from the subway. This type of convenience has encouraged an <i>incredibly</i> dense downtown (for Canada/USA anyhow) that is a destination.<p>- Discouragement of "multiple downtown" model. Here on the west coast we tend to develop smaller downtowns in multiple municipalities that are close together. Linking these by mass transit is difficult, and spreads your population thin. Toronto has a unique model in that the suburbs don't have as much of a "downtown", and all the hubbub and activity feeds into a very "mega-city" like downtown core by the lakeshore.