我的看法与蒋老师的稍有不同，不一定对，仅供网友参考：

his back retreating out of the carriage是动名词复合结构作介词after的宾语（his back为动名词的逻辑主语）。I gazed after his back retreating out of the carriage.的字面意思是“我望着他的背影出了车厢”。根据上下文可简译为：我望着他走出去。

这个句子来自于张培基英译散文赏析之《背影》（下）（https://www.hjenglish.com/new/p776124/ ）：

In crossing the railway track, he first put the tangerines on the ground, climbed down slowly and then picked them up again. When he came near the train, I hurried out to help him by the hand. After boarding the train with me, he laid all the tangerines on my overcoat, and patting the dirt off his clothes, he looked somewhat relieved and said after a while, “I must be going now. Don’t forget to write me from Beijing!” I gazed after his back retreating out of the carriage. After a few steps, he looked back at me and said, “Go back to your seat. Don’t leave your things alone.” I, however, did not go back to my seat until his figure was lost among crowds of people hurrying to and fro and no longer visible. My eyes were again wet with tears.

过铁道时，他先将橘子散放在地上，自己慢慢爬下，再抱起橘子走。到这边时，我赶紧去搀他。他和我走到车上，将橘子一股脑儿放在我的皮大衣上。于是扑扑衣上泥土，心里很轻松似的，过了一会说，“我走了；到那边来信!”我望着他走出去。他走了几步，回过头看见我，说，“进去吧，里边没人。”等他的背影混入来来往往的人里，再找不着了，我便进来坐下，我的眼泪又来了。

1、“搀”这个动词并不好翻，遇到不会翻的动词，可以选择用描述这个动作的方法来翻译，此处译为help sb by the hand

2、”怕掉身上的土“=pat the dirt off

3、“里面没人”不宜按字面直译，挖掘深层含义=“没有人照看行李“此处采用了leave alone的句式

4、“来来往往的人“=people hurrying to and fro