[Tagging] Railway routes in different directions.

[Nathan Edgars II]

If it's a continuous double-track line, it should ideally be mapped as
parallel tracks. This may not be possible due to low resolution
aerials.

It's important to note that many railroads use a signaling system that
allows a train in either direction to occupy any track, depending on
traffic conditions. Thus a fast train can pass a slower train on the
"wrong" track. This even happens at stations, where a train may
normally use one track but sometimes uses a different one; you can't
necessarily use one ride to say which side of the platform the train
stops at.

But if it's not signaled this way, and each track is one-way except
during construction or emergencies, I would tag the tracks as oneway
despite there being no legal prohibition. Motorways sometimes have
angled crossovers in the median for emergency evacuation using both
carriageways in the same direction, and during construction one
carriageway may be temporarily closed with two-way traffic on the
other. Highway law says you have to follow traffic control devices,
not the direction you "know" the way is, and railway rules are
similar: you do whatever the dispatcher or signals tell you.

All this means that you really can't have a railway route relation
that follows only one path.