Sorry to the veterans for such a noob question

I am playing that session where you have to transport carbon to a plant in the Chinese route.All goes fine until I arrive to the plant and I start to unload the carbon. It has happened to me that when I finish I notice that the train is leaving some or many of the wagons behind and I can’t couple them again to the train; I assume the joint(? Sorry for my English) is broken and I assumethe session ruined.

How to unload the carbon without problem? Sometimes I had done it very slow and it is broken anyway, is there an ideal speed or other thing?