There is a new European directive that states that every new train must have 8 bike places minimum. This doesn’t mean that every train has bike places, since directives usually come because there’s a lack of something ;) .
In General, in countries where you have a bike culture it’s quite easy. This is the case in the Belgium, The Netherlands, Denmark, Germany, but Switzerland and Austria most trains also have some bike places. Most regional trains in France have bikespaces as well. Long distance traffic -high speed traffic (TGV) is annoying to bring your bike, you almost always have to disassemble it (France-Eurostar).
The newer the train, the more easy it gets. Mostly your bike will have a place in the disabled area of the train.
In a lot of smaller stations, accessibility can be a problem. But usually bikepackers are fit enough for a bit of hassle up and down stairs.
These bike places are limited, and almost every country has different rules on bringing your bike. Mostly this comes with an extra cost, sometimes mandatory reservations, sometimes it’s forbidden to bring your bike on trains on rush hour, sometimes you need to ask it to the ticket inspector… Really dependent from country to country, operator to operator and type of train.
But without an exact idea of where you going, it’s difficult to give you concrete indications.
For Belgium: you can check your train-bike trips on this website: https://bikeontrain.belgiantrain.be/en?utm_medium=referral&utm_source=belgiantrain&utm_campaign=static&utm_content=cta_en_highlight