Dodge Junction. It is called by many as the 'source of all of Equestria's trains'; all trains either originate from there or go through it, and it is also where trains and railtracks are built.

However, various shipments of matierals, products and prototypes belonging to Iron Will Industries, Equestria's biggest developer of magic technology, have mysteriously vanished, and two of the company's employees have hired the Royal Guard to investigate

....and Rainbow Dash, Twilight Sparkle and Applejack are going to help.