Comment by mnw21cam

And because they are run so infrequently, they don't need to be fancy or efficient - just cheap and powerful. It's easier than one might imagine, but it helps if you think of scale along the lines that horsepower is roughly comparable to kilowatts, so a 200 horsepower car engine (which is small) can provide 147kW, or enough power to supply around 147 houses on average (depending on a lot of things obviously - in a climate with high heating/cooling demand it won't manage as many houses). It's not uncommon to have single diesel generators capable of generating 4MW of electricity running around on train tracks.