The suburb became increasingly urbanized as the subway lines extended.

urbanized 是动词 urbanize 的过去分词形式，表示"被城市化"的状态。urbanize 由词根 urban（城市的）和后缀 -ize（使...化）组成，意思是"使城市化"。urbanized 在这里描述郊区逐渐转变为城市的过程。