Fascinating, Chris.
I would guess that in all these cases, the railroad was in place before the airport was built. As Kevin notes, this could result in serious problems, but I would suspect that there is a high degree of caution exercised by both engine crews and pilots using these airports and the tracks crossing them.
I am unaware of any narrow gauge examples in the USA, past or present. The closest is probably the air strip at Chama, NM, which is just east of town on NM 17, and there is no way a plane would have a possible collision with a train unless it was already crash landing where it was not supposed to be. The air strip is south of NM Route 17, and the railroad is well north of NM Route 17 there.
CVM
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