My default answer is my childhood 2037.
But if that wasn't on the table I'd be running a 6 driver pre war steam switcher with all that great valve gear and rod action packed nice and tight. Like watching fancy clockwork
If got to have a wish, and a catenary system too, I always wanted the first 3phase loco & first full electric line, Ganz's Valteleena Line, It.. Basically I want a small, loop type pantagraph, electric steeple cab like some folks used to wanted the Jawn Henry, Centapede, or Big Boy. A 2-6-2, jackshaft and rod center cab?; awsome! But even a B&O steeple on bogies would work