Refactor and document railway

layers/railway/types.sql

@@ -0,0 +1,15 @@
+DO $$
+BEGIN
+    IF NOT EXISTS (SELECT 1 FROM pg_type WHERE typname = 'railway_properties') THEN
+        CREATE TYPE railway_properties AS ENUM ('bridge', 'tunnel');
+    END IF;
+END
+$$;
+
+CREATE OR REPLACE FUNCTION to_railway_properties(is_bridge boolean, is_tunnel boolean) RETURNS railway_properties AS $$
+    SELECT CASE
+         WHEN is_bridge THEN 'bridge'::railway_properties
+         WHEN is_tunnel THEN 'tunnel'::railway_properties
+        ELSE NULL
+    END;
+$$ LANGUAGE SQL IMMUTABLE;