Why would you do it? For the shear fun and of it I guess, would have about as much point in this day and age as that 5AT is ever going to have (none that I can see beyond being an exersize in engineering theory and a massive waste of money that could otherwise be spent on restoring an old engine or building more lost engines like Tornado) and possibly slightly more point as it would be more interesting to see and more of a challege to make work than just building a loco pretty much the way they have been built for decades.
Seems a little to me like the car industry, very little actual progress or new idea's just a case of "here's our latest model, it's got the same body and chassis as the last model but with a few more BHP and a satnav so we can change a few letters about in the name or ad GTR or something to the end and bingo, new model." It's still the same basic format as most cars that came before it.
No disrespect intended to the 5AT group incase anyone involved is reading just stating an opinion.
More on the topic of imaginary locomotives, this what you get if you glue two class 04 shunters together.
Still deciding the best way to go about motorising this creation.