Every train company has its own regulations about this. But I suppose you don’t plan to carry 50kg with you, so it should be fine...

Some railways have indeed MAX sizes-not weight. The tipical 1st time InterRailer is known to always have a backpack that is larger as her/him and also 2 stinky sneekers (there is no law there should be a NIke-label on these) dangling from it-other pax usually do not like that very much.

The rule is mostly quite easy: as long as you can carry it yourself and also get through narrow train doors etc its fine. Trains are no planes.

Noted with thanks  

Noted with thanks for your kind advice

Generally speaking there are no hard weight limits for luggage on most trains. Certain trains might impose sizing limits so that luggage can fit comfortably in luggage racks. On the Eurostar, for instance, luggage may not exceed 85 cm in length. You can find detailed information about each carrier's luggage restrictions on their website. It's also advisable to make sure that you can comfortably carry your luggage especially when you need to change trains, etc.