I know.
Actually, the characters are from a wee request I got. Someone asked me to write a series of stories, IN the MLP FiM universe, but with a MAJOR twist - or warp, rather:
This person (he/she wishes to remain anonymous) had read my story "The Holiday Misadventure", which was based on the 'Tales from The Other Railway" parody series made by Professor Vengeance, basically, original Thomas the Tank Engine & Friends episodes and Railway Series stories turned upside-down, featuring the Anti-Steam-Team: The Other Railway Diesels, with our beloved/abhorred "Devious" Diesel as the surly star of the show, the Anti-Thomas, if you will. Now these stories were, of course, twisted, warped, and made for an older audience than the original stories, but heck, it's a parody series.
But I'm slipping off topic.
Thing is, I really liked this "Other Railway Series", so I wrote the story. This person read it, liked it, and asked me to write an MLP: FiM series, WITH the Other Railway IN IT. I wasn't quite sure how to do this, but the guy gave me some directions - and thus, a series has started. Not as off-color humored as the Tales from the Other Railway, yet, the FiM part of the stories have also been "toughened up", creating a great balance.
And while on the pony side, we have the Elements of Harmony, on the diesel side, we have the Elements of Hatred. Now, don't judge it by it's name, as they play a key role in the world's balance. And it's not only Equestria I'm talking about.
Long story short, there will be a part where the diesels temporarily lend their powers (and their personas) to the ponies:
Diesel lends his devious demeanor and color scheme to Twilight (hence Devious Sparkle).
Iron 'Arry & Iron Bert, the scrap yard engines lend their thuggish personalities (violent & dim), and their grim look to Rainbow Dash (hence Iron Dash).
Old Stuck-Up, the posh twat chief express engine lends his extravagance and arrogance, not to mention his paintwork to Rarity (hence Old Haughty).
Spamcan, or D199, the grimy, gruff goods engine lends his less-than-flattering appearance and short-tempered working-class manners to Applejack (hence Spamjack).
Derek, the genuinely nice (if totally wussy) mix-traffic engine lends his insufferable teething troubles (and the neat smoke-screen powers that come with it) to Fluttershy (hence Camshaftshy).
Steamie, the sickeningly cheerful and scheming lunatic (latter title a courtesy of Diesel's mentorship) lends his fluorescent colors and psychotic humor to Pinkie Pie (hence Steamie Pie).
And Bowler, the chronic hypochondriac and obsessive neat-freak, uppity passenger engines lends his sensibility and snootiness, AND two-tone green color scheme to The Great and Powerful Trixie (hence The Clean and Sensible Trixie).

The reason behind the rude answer is that it was partly Diesel's moroseness talking from Twilight.
Please understand both her, and Diesel - the Other Railway's world is much more crummy, cruel and unfair than the bright fantasy that is Equestria. The only way the diesels make trough the day is by being the biggest bastards they could ever be - and Twilight is (willingly) under the influence of Diesel's devious and bitter personality - mixing with her's but still quite obviously there.