diff --git a/subway_structure.py b/subway_structure.py
index b925bb5..07af3a4 100644
--- a/subway_structure.py
+++ b/subway_structure.py
@@ -1,4 +1,5 @@
 import csv
+import itertools
 import logging
 import math
 import urllib.parse
@@ -965,11 +966,7 @@ class Route:
                         ),
                         relation,
                     )
-        if not self.stops:
-            city.error('Route has no stops', relation)
-        elif len(self.stops) == 1:
-            city.error('Route has only one stop', relation)
-        else:
+        if len(self.stops) > 1:
             self.is_circular = (
                 self.stops[0].stoparea == self.stops[-1].stoparea
             )
@@ -1382,7 +1379,7 @@ class City:
 
         self.elements = {}  # Dict el_id → el
         self.stations = defaultdict(list)  # Dict el_id → list of StopAreas
-        self.routes = {}  # Dict route_ref → route
+        self.routes = {}  # Dict route_master_ref → RouteMaster
         self.masters = {}  # Dict el_id of route → route_master
         self.stop_areas = defaultdict(
             list
@@ -1552,6 +1549,13 @@ class City:
                         continue
 
                 route = Route(el, self, master)
+                if not route.stops:
+                    self.warn('Route has no stops', el)
+                    continue
+                elif len(route.stops) == 1:
+                    self.warn('Route has only one stop', el)
+                    continue
+
                 k = el_id(master) if master else route.ref
                 if k not in self.routes:
                     self.routes[k] = RouteMaster(master)
@@ -1712,6 +1716,18 @@ class City:
                 if t not in have_return:
                     self.notice('Route does not have a return direction', rel)
 
+    def validate_route_refs(self):
+        master_refs = sorted(m.ref for m in self.routes.values())
+        for ref, group in itertools.groupby(master_refs):
+            if len(list(group)) > 1:
+                # This can occur if some routes with some ref belong to
+                # a route_master, but other with the same ref doesn't.
+                self.error("Route masters {} have the same ref".format(
+                    ', '.join(
+                        m.id for m in self.routes.values() if m.ref == ref
+                    )
+                ))
+
     def validate_lines(self):
         self.found_light_lines = len(
             [x for x in self.routes.values() if x.mode != 'subway']
@@ -1778,6 +1794,8 @@ class City:
         self.count_unused_entrances()
         self.found_interchanges = len(self.transfers)
 
+        self.validate_route_refs()
+
         if self.overground:
             self.validate_overground_lines()
         else: