In the parts shipped by rail, how many are of medium priority?
parts shipped by rail refer to l_partkey where l_shipmode = 'RAIL'; medium priority refers to o_orderpriority = '3-MEDIUM';
SELECT COUNT(T2.l_partkey) FROM orders AS T1 INNER JOIN lineitem AS T2 ON T1.o_orderkey = T2.l_orderkey WHERE T2.l_shipmode = 'RAIL' AND T1.o_orderpriority = '3-MEDIUM'