It seems to me that the only sensible direction to apply to railroads is “Unknown”. The reasoning behind this belief is that:
A: 2 trains oncoming toward each other on the same track cannot pass each other so “Two Way” does not apply.
B: Trains do go in both directions so “One Way” does not apply.
C: "Unknown is the only choice left and it has a bonus. You do not have to worry about getting the directional arrows correct!
:idea:
The only issue is that once Waze detects enough “traffic” on a railroad segment that is set to unknown, it will adjust the direction automatically. Scroll down a bit on this article. While one-way versus two-way may be life or death on a normal drivable road, trains on railroad tracks are (should be… 
) controlled in such a way to prevent collisions. The segments would most likely end up as two-way over time.
It doesn’t matter which direction is set for railroads, because they won’t be navigable. Trains aren’t using Waze, so it doesn’t matter which way trains can go on them.
Railroad segments should be set to “two-way”. This will allow them to be connected to one another more easily.
“Unknown” is not meant to be a permanent type for any road, it’s simply a placeholder until an editor or a driver tells Waze which way the road can be driven on. “Unknown” roads cannot be connected to anything, and a road that is set to one- or two-way cannot be changed to “unknown”. So the moment someone, even accidentally, sets a railroad segment to one- or two-way, the integrity of the entire network is lost.
Of course, there’s no reason to set them to one-way. Just set them to two-way, connect them, and be done with it.
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Also, make absolutely certain that the railroad segments are not connected to any road at all and set to elevation -5.