The Treasure Express was an ethereal train running on the destroyed tri-rail track between Ponyville and Dodge City. It manifested through the power leaking from the open Never Portal in Ponyville's FlimFlamCo. Heavy Industries factory, like the apparitions in the factory itself. Lozenge, Breakbeat, Double Down, and Sirocco take it to retrieve Phase 5 and 7 from New Dodge City.

Dead Red explained that it was comprised of the souls of those who had died building the railroad, those who subsequently died during derailments, then those who died pursuing the legend of the Treasure Express itself once it became an appirition. Breakbeat suspects it is also an amalgam of lesser fate spirits from The Never. Thus, the entire train is one massive Unchained Memory. It normally resembles a dilapidated monorail covered in graffiti. During the battle with Dead Red, it takes a more sinister form. It appears as if constructed from pony bodies, their limbs and faces reaching out desperately from its walls.

No alicorn in heaven could've come up with a better name. They came here looking for riches, only to see that life was the real treasure all along, right before they lost it.
Dead Red - Source, Chpt. 28