Several modernish DMUs have visited the West Somerset Railway, one of them just last week.
It was a GWR unit, and AFAIK was driven by a GWR driver with a WSR driver conducting. Numerous return trips made between Taunton and Bishops Lydeard.
Similar arrangements have been made in the past.
An HST was stored on the WSR a few years ago, and ran in passenger service on the WSR during the diesel gala.