Find the name of the train whose route runs through greatest number of stations.
SELECT t1.name FROM train AS t1 JOIN route AS t2 ON t1.id  =  t2.train_id GROUP BY t2.train_id ORDER BY count(*) DESC LIMIT 1
greatest number of stations refers to MAX(COUNT(station_id)); route runs through refers to JOIN between train and route on train id; name of the train refers to SELECT t1.name;