Devious Diesel (also known as Diesel the Rank Engine or Diesel the diesel shunter) is diesel engine (No kidding!) who's employed as yard pilot at the Big Fat Station in an unnamed City, located somewhere in Barrow-in-Furness, England.
He's a miserable little engine, who works half-heartedly on the Thin Git's railway, with (or more like against) his dastardly colleges, the other rejects from the saccharine-coated Island of Sodor.
Collectively (at least, in the city council's correctional papers), they are known as 'The Other Railway'.

Twilight Sparkle is a unicorn, who works in Ponyville's library, in the magical land of Equestria, living with her number one assistant, Spike the dragon, and living her life as Princess Celestia's protege, learning about the "Magic of Friendship" with her friends.

Alexei is disgruntled, dimension-traveling demigod, who, along with his brothers, has some important business to attend to.
And both worlds are going to be a part of it.

The fate of... Well, basically, everything is put in the hooves and the buffers of a few friendly ponies and a bunch of dastardly diesels, who have no other choice but to team up in order to save Equestria - and Existence itself - from Certain Doom.