The city's public transport system is often overcrowded during rush hour.

overcrowded 是由前缀 over-（表示"过度、超过"）和形容词 crowded（拥挤的）组成的复合形容词，字面意思是"过度拥挤的"，形容空间或场所中人数或物品超出正常容纳量的状态。