http://mct.sbb.ch/mct/en/reisemarkt/...nd-schweiz.htm

You're right, it does only mention "bicycles" but the page has a Service Hepldesk number 0900 300 300 (CHF 1.19/min.). If you're close to a station you can ask at the information (for free )

I know it can be done with a moped, a co-worker travelled all over Switzerland on one and used the SBB several times to get home after a break down.