Complex sentence: He was transferred by truck along with his unit to San Fernando, then crammed to standing room in train box cars as the Japanese overloaded them to more than double its capacity to Capas, Tarlac.
Split into simpler sentences: He was transferred by truck along with his unit to San Fernando. Then they were crammed to standing room in train box cars as the Japanese overloaded them to more than double its capacity to Capas, Tarlac.