# Why are high speed trains electric?



## abqdave (Sep 17, 2008)

The fastest steam locomotive was the British 'Mallard' that hit 126 mph in 1938. The fastest diesel locomotive is the Russian TEP80 at 147 mph. All the fastest high speed locomotives are now electric.

For regular service, in the 1960's the Japanese bullet trains ran at 125mph, as did British HST diesels. Now, a Chinese line runs at up to 350 km/h (218 mph), German ICE at up to 330 km/h (205 mph), a TGV line in France at 279 km/h (174 mph), a bullet train line at 262 km/h (163 mph), and Acela at up to 150 mph. All electrics.

Why has high speed changed from the 1960s, when electrics and diesels were equally fast, to now where all the fastest are electric? Was it a technological reason, or non-technological like politics or economics?


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## Ryan (Sep 18, 2008)

Electrics don't have to carry the weight of their fuel along for the ride.


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## PetalumaLoco (Sep 18, 2008)

I'm no expert, but I would imagine the maintenance on all-electric is a lot less than a diesel. Adding to what HokieNav said about fuel, the operating range with electric is virtually unlimited.


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## AlanB (Sep 18, 2008)

Because diesel engines can't achieve the needed output for such speeds and still have an engine that fits in the body of the engine and doesn't exceed weight limits on the wheels and rails.

Consider that Bombardier has built the only "high" speed locomotive around, in conjunction with the US DOT, and that isn't a diesel engine. It's a turbine/jet engine, as that's the only thing capable of delivering the speeds and still fitting in the body of the engine. And even then IIRC, it's only capable of something like 187 MPH or so.


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## Green Maned Lion (Sep 18, 2008)

Weren't the Turbo Trains and perhaps the Turboliners capable of that?


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## Joel N. Weber II (Sep 18, 2008)

AlanB said:


> Because diesel engines can't achieve the needed output for such speeds and still have an engine that fits in the body of the engine and doesn't exceed weight limits on the wheels and rails.


Couldn't high speed diesel trains be run with several locomotives if that's the issue, though?


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## PetalumaLoco (Sep 18, 2008)

Joel N. Weber II said:


> AlanB said:
> 
> 
> > Because diesel engines can't achieve the needed output for such speeds and still have an engine that fits in the body of the engine and doesn't exceed weight limits on the wheels and rails.
> ...


If they could, what would be the point? What's the advantage over electric?


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## Green Maned Lion (Sep 19, 2008)

Less infrastructure.


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## PetalumaLoco (Sep 19, 2008)

Green Maned Lion said:


> Less infrastructure.


I knew that. I wondered if Joel did. Or if he had another reason.


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## Joel N. Weber II (Sep 20, 2008)

PetalumaLoco said:


> Green Maned Lion said:
> 
> 
> > Less infrastructure.
> ...


I'm quite sure I knew that installing overhead power lines costs money, takes effort and is generally regarded as an undesireable expense, and also requires additional vertical clearance which can cause additional challenges. I tend to think that we ought to have overhead power lines on all long distance trackage that sees at least a few dozen trains a day anyway (along with building 5000 copies of Cape Wind to make power generation in the US much cleaner).

(Catenary powered locomotives don't work well in a yard that loads/unloads freight above the cars, which includes intermodal containers and perhaps also coal hoppers and maybe some other things. But third rail power is a good option there, and if you absolutely had to make catenary work, you might be able to get inspiration from how catenary on moveable bridges works, or have a switching locomotive on each end of a string of rail cars where the string of rail cars was longer than the catenary gap, or have a bunch of buffer cars between the locomotive and the cars that are being actively loaded/unloaded so that the locomotive never has to get too close to the catenary gap. Or maybe you just decide running a diesel in that sort of freight yard isn't a big deal in the grand scheme of things; in any rail to something else intermodal transition, the something else is likely powered by some sort of oil, whether's a tractor trailer or a ship; if coal is being hauled to a power plant, the diesel exhaust from the switching locomotive is proably tiny compared to the exhaust from the power plant in terms of environmental impact; etc.)

Also, if in fact it turns out that you can have a 200 MPH train powered by diesel locomotives, and the only major downside relative to a 79 MPH diesel train is that you need a lot more locomotives, I don't like seeing people claim that diesels ``can't'' develop enough power to have a high speed diesel powered train. To my mind, there's a big difference between a less desireable option, and something that simply cannot be done at all.


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## AlanB (Sep 20, 2008)

Joel N. Weber II said:


> Also, if in fact it turns out that you can have a 200 MPH train powered by diesel locomotives, and the only major downside relative to a 79 MPH diesel train is that you need a lot more locomotives, I don't like seeing people claim that diesels ``can't'' develop enough power to have a high speed diesel powered train. To my mind, there's a big difference between a less desireable option, and something that simply cannot be done at all.


I don't pretend to understand all the dynamics and physics involved here, but my understanding is as follows.

It's not a matter of how many engines you put into the consist. You're thinking about how much motive power or horsepower you need to move X number of cars. Instead it's a matter of how much electricity can be generated by the diesel motor to turn the traction motors at a high rate of speed. Just because you add extra engine(s) doesn't mean you can force the traction motors to turn faster. The only thing that can force the traction motor to run faster is more electricity.

And therein lies the problem. There is a maximum amount of electricity that a diesel motor can produce at a given size. The only way to get more electricity is to put in a bigger motor. Put in a bigger motor and something has to give, either you take something else out of the body of the engine or you make it longer, which now means you start to limit yourself to longer radius curves. You also start to get into a weight issue here, as to how much weight are you placing on the rails and the bridges, as you increase the size of the diesel motor.

The bottom line is that if all the engines in the consist are designed for a maximum speed of 110 MPH like the P42's, then whether you have one or twenty engines in the consist, your maximum speed is going to be 110 MPH regardless of how many cars you are pulling.


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## Neil_M (Sep 20, 2008)

abqdave said:


> For regular service, in the 1960's the Japanese bullet trains ran at 125mph, as did British HST diesels. Now, a Chinese line runs at up to 350 km/h (218 mph), German ICE at up to 330 km/h (205 mph), a TGV line in France at 279 km/h (174 mph), a bullet train line at 262 km/h (163 mph), and Acela at up to 150 mph.


The UK diesel HST didn't enter service till the mid 70's not 1960s. The normal operating speed for a TGV line is 186 mph/300 kph with a few stretches passed for 200 mph/320 kph.


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## Green Maned Lion (Sep 22, 2008)

Alan's right. The advantage of extra motors is to allow a larger train to accelerate faster and get closer to the engines theoretical maximum, as well as allow for better speed over hills.


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## Joel N. Weber II (Sep 22, 2008)

AlanB said:


> It's not a matter of how many engines you put into the consist. You're thinking about how much motive power or horsepower you need to move X number of cars. Instead it's a matter of how much electricity can be generated by the diesel motor to turn the traction motors at a high rate of speed. Just because you add extra engine(s) doesn't mean you can force the traction motors to turn faster. The only thing that can force the traction motor to run faster is more electricity.
> And therein lies the problem. There is a maximum amount of electricity that a diesel motor can produce at a given size. The only way to get more electricity is to put in a bigger motor. Put in a bigger motor and something has to give, either you take something else out of the body of the engine or you make it longer, which now means you start to limit yourself to longer radius curves. You also start to get into a weight issue here, as to how much weight are you placing on the rails and the bridges, as you increase the size of the diesel motor.


Even if that's the case, what's to stop you from removing one catenary locomotive that has two trucks with traction motors, and replacing it with three diesel locomotives, each with a prime mover, with the middle diesel having no traction motors at all, and the other two locomotives each having traction motors on one of their two trucks only, with power cables running between the locomotives much as slugs use on freight trains? You should end up with the same amount of power on the powered trucks with the three diesels that way that you would get with the catenary locomotive.

One thing I was neglecting to think about in my previous posts is that the total weight of the train does go up if you do this, and that may become a significant challenge in balancing horsepower to total weight of train. The Acela locomotives apparently were designed for four more coaches than the trainsets were built with, but if you went from two catenary locomotives to six diesels, those extra four locomotives are going to be heavier than the four coaches would have been.

I'm also assuming that the bridges are designed so that each car that passes over them can be as heavy as a passenger locomotive. Probably a safe assumption if those bridges were built for freight, and I'm not sure there are any non-subway (and similar isolated local passenger train) railroad bridges in the US that were built with the intention that they'd only be used for pasenger service.



AlanB said:


> The bottom line is that if all the engines in the consist are designed for a maximum speed of 110 MPH like the P42's, then whether you have one or twenty engines in the consist, your maximum speed is going to be 110 MPH regardless of how many cars you are pulling.


If the traction motors are designed to have a top speed of 110 MPH, then yes, that's going to be the speed limit no matter how many of them you have. But that by itself doesn't demonstrate that you can't put a 200 MPH traction motor and truck on a diesel any more than the catenary locomotives Amtrak uses for the Northeast Regional that are limited to 125 MPH demonstrate that the TGV doesn't reach 186 MPH.


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## Ryan (Sep 22, 2008)

Not to flog the dead horse, but it comes back to fuel again - if you do your 1 for 3 engine swap, you're now having to move 3x the weight, plus fuel for all 3 locomotives.

A better (but not as good as catenary, assuming that the infrastructure is there) would be 2 diesels - one w/ a traction motor geared for 110 MPH to get the train moving, and the other w/ a traction motor geared for 200 MPH that pulled electricity from the prime movers on both locomotives. The 200 MPH locomotive could never have the low end pull to get a train moving, but once it was up to a decent speed, the "high speed" locomotive could get it from 100 to 200 MPH (maybe).

The Navy is doing a lot of research into high speed diesels and more "power dense" generators that put out more juice in for a given space and weight constraints. The benefits of these would certainly be applicable to a future high speed diesel locomotive.


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## AlanB (Sep 24, 2008)

Joel N. Weber II said:


> AlanB said:
> 
> 
> > It's not a matter of how many engines you put into the consist. You're thinking about how much motive power or horsepower you need to move X number of cars. Instead it's a matter of how much electricity can be generated by the diesel motor to turn the traction motors at a high rate of speed. Just because you add extra engine(s) doesn't mean you can force the traction motors to turn faster. The only thing that can force the traction motor to run faster is more electricity.
> ...


This is probably something that PRR60 could answer better and with far greater certainty than I can, but I believe what stops you or at least makes it much harder, is the need to synchronize the power output. You can't just plug two generators in series without issue, much less three. The big power failure on the NEC a few years back was caused by this very problem. Amtrak wanted power generation from a small plant in Philly to supplement their power needs. When they tried to add that power plant to the grid, something wasn't right. So instead of providing power, the generator actually ended up drawing power off the NEC.

This caused other plants to shut down just to protect themselves from the overload being caused and led to the big blackout. But again, the bottom line here is that it's not easy to take power from one engine and combine it with another.

And then on top of that, you want to add all that extra weight and take away traction motors. Each engine currently has 4, and you want to drop it down to 2, while adding all that extra weight of a third engine. Frankly I'm not sure if you'd actually be able to move the train under those conditions. Remember the actual area of the steel wheel that touches the steel rail is probably about the size of a dime. Maybe on level ground on dry rail you might not have slippage, but I'm betting that going uphill or trying to move on wet rail, the engines would just spin their wheels.



Joel N. Weber II said:


> AlanB said:
> 
> 
> > The bottom line is that if all the engines in the consist are designed for a maximum speed of 110 MPH like the P42's, then whether you have one or twenty engines in the consist, your maximum speed is going to be 110 MPH regardless of how many cars you are pulling.
> ...


I wasn’t suggesting that you can’t put a traction motor into an engine that can’t go faster than 110 MPH. We already know that Acela’s traction motors can safely do 150 MPH, and IIRC under extreme conditions can hit 167 MPH. However there is little point in putting in a 200 MPH traction motor if you can’t supply enough power to turn the traction motor at 200 MPH.

The point of the above paragraph however was really about the idea that by adding extra locomotives, or essentially overpowering the train, that it would be able to go faster. The above paragraph was directed at that. Put another way, if it requires one P42 to move 5 coaches at 110 MPH, adding two more P42’s isn’t going to make the train move any faster than 110 MPH. And that is what you seemed to be suggesting, hence my reply.


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## Ryan (Sep 24, 2008)

AlanB said:


> This is probably something that PRR60 could answer better and with far greater certainty than I can, but I believe what stops you or at least makes it much harder, is the need to synchronize the power output. You can't just plug two generators in series without issue, much less three. The big power failure on the NEC a few years back was caused by this very problem. Amtrak wanted power generation from a small plant in Philly to supplement their power needs. When they tried to add that power plant to the grid, something wasn't right. So instead of providing power, the generator actually ended up drawing power off the NEC.
> This caused other plants to shut down just to protect themselves from the overload being caused and led to the big blackout. But again, the bottom line here is that it's not easy to take power from one engine and combine it with another.


That's certainly the case for alternating current (AC), generators need to be "in phase" or things end badly. From a quick perusal of wikipedia, the traction motors in most diesels are direct current (DC) where the phasing isn't an issue, as the voltage is constant, not cycling up and down like in AC.


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