Move railway over to imposm3 and improve z_order

layers/railway/railway.sql

@@ -0,0 +1,38 @@
+CREATE OR REPLACE FUNCTION railway_class(railway text, service text) RETURNS TEXT AS $$
+    SELECT CASE
+        WHEN railway='rail' AND service='' THEN 'rail'
+        ELSE 'minor'
+    END;
+$$ LANGUAGE SQL IMMUTABLE;
+
+CREATE OR REPLACE FUNCTION railway_brunnel(is_bridge boolean, is_tunnel boolean) RETURNS TEXT AS $$
+    SELECT CASE
+         WHEN is_bridge THEN 'bridge'
+         WHEN is_tunnel THEN 'tunnel'
+        ELSE NULL
+    END;
+$$ LANGUAGE SQL IMMUTABLE;
+
+CREATE OR REPLACE VIEW railway_z13 AS (
+    SELECT * FROM osm_railway_linestring
+    WHERE railway = 'rail' AND service=''
+);
+
+CREATE OR REPLACE VIEW railway_z14 AS (
+    SELECT * FROM osm_railway_linestring
+);
+
+CREATE OR REPLACE FUNCTION layer_railway(bbox geometry, zoom_level int)
+RETURNS TABLE(geometry geometry, class text, subclass text, brunnel text) AS $$
+    SELECT geometry,
+        railway_class(railway, service) AS class,
+        railway AS subclass,
+        railway_brunnel(is_bridge, is_tunnel) AS brunnel
+    FROM (
+        SELECT * FROM railway_z13 WHERE zoom_level = 13
+        UNION ALL
+        SELECT * FROM railway_z14 WHERE zoom_level >= 14
+    ) AS zoom_levels
+    WHERE geometry && bbox
+    ORDER BY z_order ASC;
+$$ LANGUAGE SQL IMMUTABLE;