D,
Unfortunately, not many if you want to travel only by train. You can get from Boston to Buffalo pretty easily, but not northwest to Toronto. Here's why:
-Boston has only one train a day heading west to Albany, Buffalo, and then Chicago (the Lake Shore Limited, train 449).
-Amtrak only has one train that serves Toronto, the Maple Leaf, which runs New York-Albany-Buffalo-Niagara Falls-Toronto.
The problem lies in the fact that the Lake Shore Limited hits Albany and Buffalo well after the Maple Leaf, so you can't change trains from the LSL to the Maple Leaf without an overnight stay in a city like Albany or Buffalo.
Your best bet, honestly, is to drive to Albany, park there, and catch the Maple Leaf out at 10:00 AM. Arrives Toronto that night at 7:47 PM.
-Rafi