CREATE TABLE table_21716139_1 (destination VARCHAR, frequency VARCHAR, train_name VARCHAR)

How many destinations have a weekly frequency and are named AC Express?
SELECT COUNT(destination) FROM table_21716139_1 WHERE frequency = "Weekly" AND train_name = "AC Express"
SELECT COUNT("destination") FROM "table_21716139_1" WHERE "AC Express" = "train_name" AND "Weekly" = "frequency"