Your suitcase size sounds fine. It's not THAT big and it will leave you room in case you decide to do some shopping (very tempting in Italy). In the end, do what YOU feel comfortable with. I routinely go on trains all across Europe with a suitcase bigger than 22" and have never had a problem maneuvering it myself and I am no Amazon (I even hauled around three suitcases with a broken finger). Moreover, I never felt guilty or ashamed about it (some posters seem to think that traveling with more than carryon size luggage is a sin that requires penance).
FYI, if you are traveling in first class, you will generally find more luggage room.

My parents travelled with us in December, and they both had bags bigger than that, that expanded even further. Sometimes it would fit overhead and sometimes it didn't. But, when it wouldn't fit in the overhead they stored it in the area between the train coaches.

Ah, but, Sandi, that was in December. In the high season, the space at the end of the car for luggage can fill up. There's a little more room in first class.

I say go with what you already own! One suitcase and one smallish backpack sounds just fine for a 5 week trip on planes & trains. Just don't stuff either one, so you keep the weight as light as possible.

Have fun, lucky you!

we traveled only in first class and the overhead deminsions were different on some trains.

Do what is right for you. I tend to overpack...so be it. I pack my "big" bag and "small" carryon and then practice slinging it around. Up into the van, up and down steps, etc. If I'm willing to take full responsibility for the weight, then I'm merrily on my way and who cares what someone else thinks or would do. If its too heavy, then I open up the suitcase and debate on what to leave behind. As long as you can (and are willing) to sling around your own suitcase, then you are just fine.

Go with your 25" wheelie. You may want to pack an extra soft-sided duffle to bring back souvenirs.

Don't take a "wheelie" backpack, especially one with those useless little zip-off bags. The frame needed to support the handle/wheels structure, and the wheels (a) add weight that could otherwise be devoted to belongings, and (b) will make the backpack uncomfortable to wear. Find a large daypack - about 30-35 litres, preferably one with only one main compartment (although with as many narrow front pockets as you want). The North Face and Victorinox have good ones.

And I'll be one of the people who says your 25" suitcase is probably too big. Why do you need to take so much stuff for a 5-week trip? Design a wardrobe based on layers and multi-purpose items, and you should be able to reduce the volume of stuff you pack considerably. No one is going to care if you wear the same clothes frequently (and you can wash them when you get there, if you need to).

I'd suggest that you load your 25" suitcase up with everything you plan to take (including shoes, coats and other heavy things), load up your daypack with the guidebooks you think you'll need, plus a big bottle of water, etc and then jog a few miles, while incorporating some sprints on stairs and overhead lifting. Do that for an hour or so as the wheels and frame of your wheelie daypack dig into your spine, then go home and buy a smaller suitcase and a comfortable daypack.

I still think aisle width can be a KEY factor, if your bag wheels sideways. The fast trains that most tourists take in Italy (2nd cl) seems something like 23" wide aisles, or maybe 22". Imagine getting down these aisles, especially with even 1 of 2 bags oversize - it's a horror show I have seen. I think their slow trains have wider aisles, as well as fast trains in other countries.

I think it's ebags.com that can offer you bags sculptured exactly as you wish due to wide selection... cheaply and quickly. Either a bag wheeling in the narrow direction is better, or what I prefer is the no wheel, no-gratuitious-strap backback. Just a crisp, presentable, space-efficient rectangle with 2 hideaway straps and no other heavy junk like wheels or frame.