Like a battleship, I'd guess. They'd likely have had water-tube boilers heated by petroleum flames or something, since shoveling coal might become somewhat unfeasible at that scale. Unless you, of course, have multiple smaller boilers, which would however be a heavier and more manpower-intensive solution. The distinction between those two problems being that lost efficiency hasn't historically been such a big of a deal for American railway companies, unlike minimizing the required number of crew, considering they've time and time again chosen inefficient designs and unsafe practices in the hunt for their Holy Grail of the one-man train... I mean, when the costs of wasted coal and fuel surpass the wages of having an additional man, you're likely just out to spite unions.
What I'm trying to say is that pseudo-automation of such kind would not just have been more practical at that scale, but closer in alignment with common corporate principles.