diff --git a/agenda/trip.py b/agenda/trip.py
index 06fbf2a..db148e9 100644
--- a/agenda/trip.py
+++ b/agenda/trip.py
@@ -9,6 +9,12 @@ from agenda import travel
 from agenda.types import StrDict, Trip
 
 
+class UnknownStation(Exception):
+    """Unknown station."""
+
+    pass
+
+
 def load_travel(travel_type: str, data_dir: str) -> list[StrDict]:
     """Read flight and train journeys."""
     items = travel.parse_yaml(travel_type + "s", data_dir)
@@ -24,8 +30,10 @@ def load_trains(data_dir: str) -> list[StrDict]:
     by_name = {station["name"]: station for station in stations}
 
     for train in trains:
-        assert train["from"] in by_name
-        assert train["to"] in by_name
+        if train["from"] not in by_name:
+            raise UnknownStation(train["from"])
+        if train["to"] not in by_name:
+            raise UnknownStation(train["to"])
         train["from_station"] = by_name[train["from"]]
         train["to_station"] = by_name[train["to"]]