Hello drushnik
Travelling in a wheelchair on public transport in Switzerland is now easier than ever. Although some trains and public transport vehicles are not wheelchair accessible yet, most trains and buses do provide platform level entry. In trains where the wheelchair compartment is located in 1st class for design purposes, you are welcome to travel in 1st class although holding a 2nd class ticket.
In Switzerland, a law has been enacted to guarantee that individuals with reduced mobility can travel more independently in the future. SBB and other public transport companies are about to convert all their station platforms to make platform level access possible. To modify all stations takes time and is a costly undertake.
In its online timetable, SBB provides the opportunity to specifically search for connections that are suitable for individuals with limited mobility. In some cases, assistance from railway staff may be required; this can be ordered by phone from SBB Handicap with sufficient lead time: Contact Center Handicap | SBB
You can find connections suitable for PRMs in the SBB timetable by enabling the respective filter, that is part of the Advanced search. See Barrier-free travel options:

Please let us know should you need additional information.
Kind regards,
Roland