There seems to be a strange issue with Oxford (UK) station at the moment where you can’t walk through the front doors. If you create a simple route from near the station to the station itself, you’ll get routed around the back of the station and over the railway bridge instead of the shortest route through the front. The route changes depending exactly where you place the finish marker inside the station building, but it appears that the pathways through the station itself are inaccessible.
Normally routing takes you to the closest point that is routable: I suspect that this is what the routers are doing.
The mapping of platforms looks very complicated and confusing: they belong to both multipolygons and public transport polygons. This needs to be tidied up.
The fundamental issue is likely to be that railway platforms and any highway with an area=yes tag are not routable. If you want to route you are likely to need to do some of the following:
Map platforms as ways (not areas) and also tagged with highway=footway. These may also be tagged with the public_transport tags
Have distinct footways to the base of stairs, through the booking hall etc.
Get rid of all unnecessary multipolygons (anything which does not have a hole in them.
Add railway=platform areas for rendering purposes.
In practice all of these are ‘tagging for the router’. There are more advanced & better ways to map the inside of stations but as yet I dont think we use them in the UK.