Not gonna get into an in-depth discussion about economics. But there is a huge difference between people taking the train and people driving. When you take the train you don't have extra room for groceries, extra room to put items purchased while shopping, you're not paying for gas, you're not paying for a car, you're not getting it maintained. All those things plus many more stimulate the economy and create jobs. Not to mention driving gives you the freedom to go wherever you want (within reason).
And as for agricultural land, we've pretty much developed all the land that is able to sustain agriculture (the majority being in the lower mainland and in northern BC east of the rockies). None of it needs to get removed though. The sprawl can take place deeper into the forests and hills.