Welcome to Fluther.
As noted by others, and as you are undoubtedly learning as you study English, it can be confusing!
That is, you probably hadn’t even thought about the idea of “a train” being anything other than a locomotive engine pulling (or pushing) iron-wheeled cars on a railroad track. Look at the dictionary definition of the word ‘train’ to see other possibilities.
After that, or course, “huge” and “big” can relate to other dimensions of the (railroad) train’s size. “Long” is more specific – if that’s what you mean – and in that case probably best.