Am 09.10.2019 um 21:32 schrieb csipromo via Groups.Io:

I have never heard the term SIM for trains.

Look at the official document
Designation of trains 2020 (PDF, 886 KB)
at
<https://company.sbb.ch/en/sbb-as-business-partner/services-rus/onestopshop/track-access.html>

There you will find that train numbers 43500-43799 and 63400-63799
are SIM trains. These numbers are recognised by signal box software,
so that these trains will always remain on tracks where they are
allowed.

Markus, Gürbetal