NYC–Montreal and NYC–Toronto overland

Seat61 has a good page covering these three routes. You basically have one daytime train a day each way, between New York and Toronto (The Maple Leaf) and New York to Montreal (Adirondack). Journey time is around 12 hours, with the trains during the morning and arriving in the evening. It looks like there's nice scenery for much of it, but it does take a lot longer than flying.

Between Montreal and Toronto, you've roughly 5-6 trains per day each way. Journey time is about 5 hours, and there are some lovely views (especially if you're on the south side of the carriage, so you can look over the lakes and rivers)

If you are looking for the cheapest option you can try to share a ride from NYC to Montreal or NYC to Toronto. There is also a Facebook group NYC-Montreal. However, the train might be more reliable and i would prefer it if you have a tight schedule, or a flight to catch.

I have never taken the train between those routes, but on the west coast between Vancouver and Seattle someone (here in this forum?) recommended the train over the greyhound service because apparently the border crossing is much smoother. Basically, you clear immigration at the train station, then they re-check your passport at the border.