How many cars are there on train no.1?
train no.1 refers to train_id = 1
SELECT COUNT(id) FROM cars WHERE train_id = 1
database name:db_id trains
cars table cars Cols: id dtype integer, train id dtype integer, position dtype integer, shape dtype text, length dtype text, sides dtype text, roof dtype text, wheels dtype integer, load_shape dtype text, load number dtype integer, 
trains table trains Cols: id dtype integer, direction dtype text, 
