note that if this tag describes in use feature that makes sense then it can and should be documented at Tag:route=shuttle train - OpenStreetMap Wiki (if anyone needs help then feel free to ask, but Creating a page describing key or value - OpenStreetMap Wiki may help)
It is also fine to take over proposal (or copy it) and send it to RFC and later vote.
See also Route=shuttle_train - deprecation of 13/11 year old (never activated) proposal
(It is also possible that for whatever reason it was a mistake - and I definitely sometimes make mistakes - in such case you can revert my edit on OSM wiki)
((I reverted it for now))