Show the builders of the railways.
select builder from railway
List the railway id and the builder for each one.
select t2.railway_id, t1.builder from railway as t1 join train as t2 on t1.railway_id = t2.railway_id
Among those results, keep the one that associated with the most trains.
select t2.railway_id, t1.builder from railway as t1 join train as t2 on t1.railway_id = t2.railway_id group by t2.railway_id order by count(*) desc limit 1