A freight train is long, heavy and hard to stop. Dream books read it as carrying great burdens, unstoppable momentum or something coming that cannot easily be turned.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.