Funny the things you discover when looking for something else.

Meet, for example, a General Electric diesel electric locomotive.

They’re sold in big numbers to North American railways and sent further afield as well, like this one. It’s a Class 70 as used in the UK and with more conventional (to my eyes) design with double ends and cab doors on the sides.

One of them had to be sent back. See why:

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Specs vary, but generally you get a really big engine with about 4,500 bhp (if emissions worry you) and about 6,000 (if they don’t). More importantly to users, it’ll pull things really hard, with a tractive effort of nearly 900 kN.

GE and EMD locos are the only items of US stock that you’re ever likely to see on European rails, diesel being a bit of a niche on this side of the pond which local makers don’t necessarily trouble themselves with.

Meet, on the other hand, a Siemens Vectron, intended for the Central and Northern European markets.

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Advanced readers will already know why it’s sitting on yellow frames, but as a clue it’s made in Munich and being taken to Finland. There’s something unusual about the rails in Finland.

Vectrons are almost all electric. Siemens offer a diesel but it took until last month for anybody to order one. Electric traction has many advantages, but one is that you can fit very powerful motors as the limiting factor is now the available current, not the size of engine you can fit in. The more powerful variant has 6.4 MW or about 8,600 bhp so allowing for power losses between engine and motors it’s about twice as powerful as the clean GE unit. So, it must have a simply enormous tractive effort, right? Nope, it’s about 300 kN.

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So, there you have it. The fancy, complicated (actually simpler, but never mind) Yoorpean has all this power and can’t do anything with it. ‘Murica! Freedom! Trump!

Except none of that’s true.

Each loco is designed for different conditions. In North America (and Australia, South Africa and other places) long distance railways are used mainly for freight. A GE loco and a few of its friends are used to haul enormously long and heavy trains a long way and as they don’t have to worry over much about holding up passenger services they don’t have to go very fast.

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In Europe freight is a much smaller proportion of rail traffic and becoming even less so, partially because of the demise of coal power which was usually supplied by rail. Freight traffic therefore has to keep up or passenger operators won’t be happy. Freight trains are also shorter and lighter because destinations are likely to be nearer and you don’t want to go marshalling an enormous train too often You therefore don’t want tractive effort to the same extent, but you do want speed and to get that you need power.

Let’s look at the specs. The GE loco has three axle bogies (railway people talk about Co Co), the Siemens has two axle ( Bo Bo, and yes single axle would be Ao Ao). Twelve rather than eight wheel drive then, and the GE has a max weight of about 190 tonnes compared to about 90 so each of those wheels has a lot of weight on them. Then there’s gearing. GE gear their unit for 75 mph/120 kmh weight permitting while Siemens offer 160 or 200 kmh and intend their machine do those speeds with a heavy load.

So now we understand. Haul a long train slowly or a short one quickly.