When the lockers are all full
Long weekends, event days, big stations in the morning — the lockers go all at once. This site lists how many are installed, not how many are free, so arriving to find them all in use does happen. Here is what to try, best first.
1. Try elsewhere in the same station
The bigger the station, the more scattered the lockers. The ones by the gates go first, so a bank down a side exit or in an underground passage is often still open. The station pages here list every location — aim for the inconspicuous ones.
2. Use a staffed luggage counter
Major stations have staffed baggage rooms. They cost more than a locker but take almost any size, which matters for a large suitcase. Same-day collection is the norm.
3. Go one station over
The station next to a tourist spot fills up; the one after it often does not. If it is a few minutes on the train, that usually beats searching.
4. Check the station building
Shopping complexes attached to stations have their own lockers. They rarely appear on station maps, which is exactly why they are still free.