There is more than one opposite of crowded. You can use 'uncrowded' (not containing many), 'empty', 'deserted',. Empty is best for interior space. 'The train was crowded'. 'The train was empty'. Deserted is best for large open spaces. 'The town was crowded' 'The town was deserted'. Comparative of crowded = more crowded. Comparative of empty = emptier. Comparative of deserted = more deserted.