I absolutely ADORE taking the train from Montreal to various places in North America, and I 514% have to agree with Insider for ranking the Amtrak's Adirondack as most beautiful in North America! I used to take it EVERY year to visit my family in New York when I was younger, and trust me it is STUNNING!
The route begins at Montreal then Saratoga Springs and it ends in New York (and vise-versa). It is a 10-hour ride (one way) that takes you through the most beautiful places in North America! You can leave Montreal in the morning and arrive in New York by dinner time.
The train takes you through some of the cutest "hidden towns" that are lined up with stunning mansions and of course along the beautiful Hudson River Valley where you can expect to see nature in all its glory and a variety of wildlife such as beavers, deer, and so on.
In addition, as you look out the window for 10 hours straight (like I did as a kid) at the pure beauty, you will see the Adirondack Mountains, the Palisades, West Point Academy's riverfront stone battlements, and the Bannerman's Castle ruins. Oh, and if you forget to pack some snacks, not to worry since the train is equipped with a cafe car!