Fantastic idea! I'd love to have functioning railroad crossings on paths.
We can already make fake railroad crossings but guests won't stop when a train crosses the path.
Perhaps this can be implemented by making the curbs that block guests triggerable as active/disabled for x amount of time by an approaching train? The amount of time would have to be customisable in order to fine-tune it depending on how long guests will have to be blocked from crossing the railroad. The curbs would have to be hidden beneath the path, so this would take some work, but it would also certainly be nice to have in the game.