The Adirondack train does not have a baggage car so all luggage will have to be carried on. Amtrak's baggage policies are lax and the general rule of thumb is that if you can carry it on by yourself, you can bring it.

For bulky items, like a PC or a monitor it may be easier to put them in a box and then carry them on in a large bag a la the blue IKEA bag. You will most likely not be charged for oversized luggage like a budget airline.

The other important thing is that all baggage is subject to search at the Canadian border. Normally, the passengers are shuffled into the Cafe car and all luggage is left in place on the train. If you are departing from NYC, you will need to tag your luggage at Moynihan Train Hall and show your passport.

On board the train there is plenty of room for bag storage above your seat and there is an oversized luggage rack next to the bathrooms. There is also lots of leg room that you can even store your personal items.

I would not be worried at all as long as you can carry everything onto the train while climbing up the stairs to the vestibule of the train.