SELECT ObjectNumber FROM railway WHERE Railway_ID NOT IN (SELECT Railway_ID FROM train)
List the object number of railways that do not have any trains.
[Schema (values) (types)]: | railway |  railway : railway_id (text) , railway (number) , builder (text) , built (text) , wheels (text) , location (text) , objectnumber (text) | train : train_id (text) , train_num (number) , name (text) , from (text) , arrival (text) , railway_id (text) | manager : manager_id (text) , name (number) , country (text) , working_year_starts (text) , age (text) , level (text) | railway_manage : railway_id (text) , manager_id (number) , from_year (text);