yes, I apologize. Which object number railways that do not have any trains?
select objectnumber from railway where railway_id not in ( select railway_id from train )
sorry, yes.
select objectnumber from railway where railway_id not in ( select railway_id from train ) and location = "York"
which object numbers do have trains?
select objectnumber from railway where railway_id in ( select railway_id from train )
who is the builder of 
Object Number 1975-7006?
select builder from railway where objectnumber = "1975-7006"