I bought a lightweight bag to travel to and from Germany by train last year. The bag had no padding, and would take the bike with wheels removed. It weighed less than 500g I think, and I strapped it to the lower part of the bike’s downtube during the riding.
Within the trains it wouldn’t fit fully in the luggage areas, but it didn’t extend enough to cause any issues.
I’m planning to use it again this year for trips to Switzerland and SW France. I’d say these types of bag are good when you are moving it around onto trains, but I’d be tempted to add some padding to key areas if others will handle it ... bagged up clothing might be suitable. Just be careful not to put too much in, as the bag with bike is hefty to lug around.