Sections of railway are dropped during processing by mkgmap. The cause
of this is that the sections are tagged layer=-1 even though they are
not in a tunnel.
The question which arises in a situation like this is: is the tagging
wrong, or should the behaviour of mkgmap be changed? At first sight it
seemed simple to me. A railway which is neither on a bridge nor in a
tunnel should be in the default layer (layer 0). Any crossing ways on a
bridge or in a tunnel should have the layer set to 1 or -1 respectively,
except for multi-level civil engineering works. However, the contrary
case is put in various OSM mailing lists. If a railway is in a cutting
and crossed by several bridges, it is simpler and it also makes sense to
put the railway in layer -1 and leave the bridges in the default layer.
In my opinion, tagging with layer=-1 in that particular case is not
wrong, but tagging long stretches with layer=-1 would be wrong. Looking
at r1445 in the mailing list, it was clearly the intention to hide
underground railways. In light of that, I suggest that the mkgmap
default style might be modified to test for !(tunnel=yes) instead of
!(layer<0), for all four types of railway.
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