> Hm, seems like I realized why there was a dedicated route=shuttle_train way.

Yeah, another reason - but still not valid reason.

I've discussed this topic with the maintainer of osm2po and I've come to the conclusion that valid transfer points from highway=* to railway=rail should be - and are - defined explicitly:

- for motor_vehicles, ...: https://wiki.openstreetmap.org/wiki/OpenRailwayMap/Tagging#Car_shuttle also tagged as public_transport=stop_position

 - for bicycles and others : public_transport=stop_position on the rail, public_transport=platform next to it for the bicycles

So to have a positive (and not a never exhaustive negative) list of transfer points