I had a mental picture of someone accidently taking your MIL luggage from the train, unzipping it at their destination and finding Depends.

I took a 25 "-26" on a train in France and stored it near the door with no problem. I pushed it down the stairs when I got off instead of lifting it. I don't remember seeing carts in the train stations in Marseille or in Rome. On a different trip - different suitcase, I took a train from Rome and put my 24" in the overhead. I don't think anything larger would fit so would have to be stored at the end of the carriage.

"...so they work a bit better than ziplok's."

I have both and don't think the vacuum bags are worth the expense. You can press a lot of air out of zip-lock.

Does this 25" suitcase have wheels? If yes, I don't see a problem. Or that you'll need to look for trolleys (as long as each person's suitcase has wheels).