That depends a lot on the countries and trains. There is no general answer, although there should at least be one accessible toilet on a train.

By a kind of EU-law trains MUST be accessible to anyone-also with handicap. However, there is an exception that older trainsets do not need a costly rebuilt if they were bought 20 yrs ago without it. The other planners (best is bahn.com for overall, far best is of course always that of country/railway concerned) should give details. BUT in many countries you have to call for assistence and this has to be done 24/48 or whatever hrs advance and confirmed. The real modern trains are same level as pletform and can thus be accessed without help. Does ot really matter if its hi-speed or not.

Most western countries have some kind of club/association for people in same situation and these will be better informed.

If you are planning on travelling with a wheelchair and you need to sit in the chair on the train you need to, at least in Sweden, to make sure that such a space is available on the train you plan to take and that you book assistance to get on/off the train.

Here is information from Interrail/Eurail on how to book assistance:

https://www.interrail.eu/en/support/interested-in-interrailing/travelling-with-a-caregiver