The rail tonnage figures fell by about 40% in the 1980s and 1990s but then bounced back by the beginning of the new century.

bounced 是动词 bounce 的过去式，bounce 本意是"弹跳、反弹"，这里比喻为"回升、恢复"。 back 在这里是副词，表示"回原处、恢复原状"。 bounced back 是一个常用短语，意思是"反弹、恢复"，指在下降或衰退后重新回升到原来的水平或状态。