I think you must be confused by the meaning of "火车道上压过腿", because I think "压" doesn't mean "squash" here, instead it has a similar meaning with "stretch", which is derived from the phrase "压腿(leg press)". But I don't think "火车道上压过腿" has some concrete meaning here, it just helps with the rhyme. The whole sentence just means "I have been through a lot", in a more hilarious way.

走南闖北 : 往來各地、到過很多地方。 如：「他走南闖北多年，閱歷不少。」 火車道上壓過腿 : 屬於 誇飾修辭法 誇飾 : 語文中誇張鋪飾，超過了客觀事實的修辭法，稱為「誇飾」。 說話者或許閱歷豐富 但被火車壓過腿 可能就誇張了 無非是 說話者的 自吹自擂 反正 騙死人也無需償命 我得說 中國說唱藝術表演者的語言文字運用相當生動活潑 而且 句句押韻 同時增加了韻律趣味 走过南闯过 "北"，火车道上压过 "腿"。 长江黄河喝过 "水"，还跟傻子亲过 "嘴"。 春风 "吹"，战鼓 "擂"，赵老大我怕过 "谁" 呀？！

吹牛代表自己阅历丰富

当楼主暮年告老归乡的时候，一定会吸引着故乡的小伙伴，他们会拿着凳子倾听你的传奇故事，那个时候你也许就会由衷的开始讲述你“走过南，闯过北，火车道上压过腿”的美妙传奇故事。

It is an issue you are,bragging about something.