Let’s explore something unfamiliar.

This thing, to be exact.

It’s a little choo choo locomotive designed for a very specific purpose. It’s a Gmeinder D75BB and has about 750 bhp to play with. It weighs about 80 tonnes, but a truck with this kind of power to weight ratio could proceed quite quickly on level ground.

So, what’s the maximum speed of our little (OK, it’s 80 tonnes but these things are relative) loco?

Twenty five. No, not 25 mph. 25 km/h. A fast runner could pass it out.

Yes, there’s a reason for this lack of speed. It’s designed to shift freight wagons around a yard (it’s what’s called a shunter here but there are other terms) and there’s no need to get up any speed. Instead it uses very low gearing to move really heavy things without having to have a big, expensive, thirsty engine. With enough traction and low enough gearing, you can move the world.

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There’s something else interesting for the technically minded. It has a hydrodynamic transmission with two gears, one which takes you forwards and one which takes you backwards. It has however two speeds in each direction. How so? Each gear is driven by two torque converters, one of which slips a lot (low gear) and one which doesn’t (high gear), so two each forward and reverse. To change gear you drain one and fill the other. To change direction you just fill the reverse converters. You can do this at speed so you can put your loco into reverse in order to provide extra braking effort and save wear on the brakes. Clever, eh?