In general, razed/abandoned railways can be kept in OSM when remains are
visible (= on the ground rule). When something else has been built over
the railway, I would always delete it, there’s no room in OSM for purely
historical information (this is what OpenHistoricalMap is for). However,
even with that being the case, I would not “hunt” for razed/abandoned
railways and try to get rid of them on a large scale - I’d only remove
them where they interfere with my mapping. On the other hand if someone
were to engage in a large-scale endeavour to add abandoned/razed
railways I would stop them.

Regarding planned infrastructure, that depends on the planning stage.
In Germany we tend to only allow planned stuff in OSM when the plans are
at a stage where the thing is funded and will definitely be built in
that location to a defined schedule - and not when the plans are at a
stage where alternatives are discussed or funding hasn’t yet been
allocated. But other regional communities might have different approaches.

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