Refactor and document railway

layers/railway/railway.yaml

@@ -1,11 +1,24 @@
 layer:
   id: "railway"
-  description: Rail
+  description: |
+      The `railway` layer contains linestrings marking tracks from [OSM Railways](http://wiki.openstreetmap.org/wiki/Railways).
+      It contains tracks for [passenger and freight trains]() and smaller tracks for [Trams](http://wiki.openstreetmap.org/wiki/Tag:railway%3Dtram) or [similar](http://wiki.openstreetmap.org/wiki/Tag:railway%3Dlight_rail) vehicles. But also tracks for [subways](http://wiki.openstreetmap.org/wiki/Tag:railway%3Dsubway), [narrow-gauge trains](http://wiki.openstreetmap.org/wiki/Tag:railway%3Dnarrow_gauge) or [historic trains](http://wiki.openstreetmap.org/wiki/Tag:railway%3Dpreserved).
+      Non mainline tracks (marked with class `minor_rail`) used for [storage of trains](http://wiki.openstreetmap.org/wiki/Tag:service%3Dyard) and [maintenance](http://wiki.openstreetmap.org/wiki/Tag:service%3Dsiding) are contained in the highest zoom levels and should be styled more subtle than the mainline tracks with class `rail`.
+  fields:
+    class: |
+        Divides the track into mainline tracks (class `rail`) and less important tracks
+        used for maintenance (class `minor_rail`).
+    subclass: |
+        Original value of the [`railway`](http://wiki.openstreetmap.org/wiki/Key:railway) can be one of
+        `rail`, `light_rail`, `subway`, `narrow_gauge`, `preserved`, `tram`.
+    properties: |
+        Additional properties describing the nature of tracks. Can be either `bridge` or `tunnel`.
   buffer_size: 4
   datasource:
     geometry_field: geometry
-    query: (SELECT geometry, class, subclass, brunnel FROM layer_railway(!bbox!, z(!scale_denominator!))) AS t
+    query: (SELECT geometry, class, subclass, properties::text FROM layer_railway(!bbox!, z(!scale_denominator!))) AS t
 schema:
+  - ./types.sql
   - ./railway.sql
 datasources:
   - type: imposm3