I was in France last month and while there took a trip to Reims by train. This involved the French rail network’s pride and joy, a TGV high speed train which travels on a matching LGV or high speed line. It reached just over 300 kmh or 190 odd mph during the short journey.

Here’s the train used.

It has eight coaches topped and tailed by a pair of semi permanently coupled power cars. It’s electric, which put me to wondering. Could you replace the electrons by diesel?

This is a GE diesel electric loco.

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It’s a big, ugly looking thing and it’s American so it’s obviously much more powerful than those poncy French electric things. Isn’t it? Nope. Well, it’s almost as powerful, isn’t it? Nope. In fact, those fancy electrics are in a completely different league. Each of them puts 4.8MW or about 6,450 bhp to the wheels. A diesel loco produces about 4,000 bhp at the flywheel and something like 3,600 bhp at the driving wheels if you assume 90% efficiency for electric drive which is generous. We won’t be replacing two electric power cars by two diesels then. So let’s try four and do some sums.

A TGV weighs 383 tonnes empty. I don’t know how much of this relates to the power cars but a conventional electric loco weighs about 80 tonnes so let’s take that. A diesel like the one above is at least 100 tonnes. We take off 160 tonnes of electric power then and add 400 tonnes of diesel. We now have 14,400 wheel hp compared to 12,900 before. Trouble is though the power to weight ratio has gone haywire. We began with 12,900 bhp, 383 tonnes and 33.7 bhp/tonne and have regressed to 14,400 bhp, 623 tonnes and 23.1 bhp/tonne. Six locos? No, that gives 26.2 bhp/tonne. Eight? That would be 28.1. You could still achieve your desired 300 kmh on diesel, but not in enough time to be useful because poor acceleration.

So there you have it. If you want to go really fast on rails, you’ll need mains electricity. The fastest diesel services that I can think of reach 200 kmh or about 125 mph and they need a long run to get there..