Question: Show the id and builder of the railway that are associated with the most trains.
Schema: create table train (railway_id varchar, PRIMARY KEY (railway_id)); create table railway (builder varchar, railway_id varchar, PRIMARY KEY (builder))
SQL: 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