These can be aliased together: train_tracks, railroad_tracks and railroad. The most dominant usage seems to be railroad_tracks.
It appears to me that crossing can be marked as ambiguous and its posts moved into crosswalk or railroad_crossing as appropriate. Or it can be kept as an umbrella tag for both cases.
I am not sure about the distinction between railroad_crossing vs railway_signal. There seems to be a lot of overlap, so perhaps they can be collapsed together. Or does railway_signal mean any kind of signal or semaphore, regardless of whether it is situated at a level crossing or not? (e.g. post #610450).
As a side note, rails contains both railroad_tracks and railing, I am presently cleaning that up and will mark it as ambiguous (it is probably unsafe to alias).
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