Sturdy is not so much an issue on the trains. You are the one handling the luggage so it's up to you whether it gets mishandled or not.
Size is an issue. There are different storage possibilities for your luggage on the train:
The overhead compartment
The racks at the end of each car
The space between two seats that are back to back in certain cars
Underseat space
With all of these it will help immensely to have a suitcase not bigger than 24". This will also help getting it into the car and down the aisle to your seat (if you have a huge suitcase your storage options are limited to the racks (often full) and the between seat spaces (often still empty because people aren't smart enough to figure out that option)).
It also helps to have a spinner because that will be easier to push down the aisle. So I'd go with spinners no bigger than 24". Soft shell. If you can find some with double wheels, ideal. Briggs Riley comes to mind but they aren't particularly lightweight.
If you check the sticky on top of this forum, there will be more suggestions on really lightweight luggage. But only few of those will be wheeled.
How fit are you guys? I think one of the ideal options for such a trip is a backpack or a convertible backpack. You are supposed to take the backpack off when you enter the train or public transportation but the bag will still be easier to handle and have a better volume to weight ratio than a wheeled bag.
I also wouldn't use three suitcases (provided you are only two people). Pulling two suitcases on one person is just not a good idea. A 24" suitcase and a little duffle/ cabin bag should fully do the trick. The small bag can then ride on top of the suitcase. If you buy an expendable suitcase and expendable duffel you have a double reserve for things you might bring back from Europe (shopping in Milan and Paris is great and you are there during the sale). I'd recommend to actually not go with a full suitcase.
As for brands or concrete recs, just check the specs and see which one is light. A light 24" suitcase with wheels should not be more than 10lbs.
Briggs Riley and Eagle Creek are great for warranty on the wheeled pieces. So is Travelpro Platinum. None of which are particularly light. > Sturdy build, good warranty = not very light, not cheap.
If you really want to go light, you will have to forgo wheels. In which case the world is your oyster and there are equally plenty of great options available. Faves here are Tom Bihn, Red Oxx, BR, Eagle Creek, Patagonia.
Till