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May 2016
 

The Bellinghman
Date: 2009-12-23 15:49
Subject: Old tech saves the day
Security: Public
As seen here: Passengers rescued by Tornado.

It's rather wonderful in its way that when the electric-driven trains were being disabled by ice and snow, a good old-fashioned A1 Pacific steam loco could carry the stranded passengers to London.

He said: "It was a nice way to finish for Christmas, though I think some of the rescued passengers didn't realise they'd even been travelling on a steam train until they got off."

Hmm. I wonder what they thought the plume flowing back along the train from the front was, then.

(Steam trains are wondrous things, but I wouldn't really want them back in bulk. Electrics can run in almost any weather, as the Swiss can demonstrate, and they don't have to run on hydrocarbons.)
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User: knell
Date: 2009-12-23 16:00 (UTC)
Subject: (no subject)
Well, diesels run fine under these circumstances too...
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User: furrfu
Date: 2009-12-23 16:07 (UTC)
Subject: (no subject)
But are a whole lot dirtier than electric, still.
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The Bellinghman
User: bellinghman
Date: 2009-12-23 16:19 (UTC)
Subject: (no subject)
When we were riding the Canadian from Toronto to Vancouver this summer, we noted just how much dirt was coming out the top of the engines. Yuck. I'm sure they could do something to clean it up a bit.

One of our co-passengers was a local representative (or worked for one) in Toronto. Toronto is thinking of opening a new line out to the airport (the Union-Pearson Rail Link), running lots of trains. They'll be diesels. This is apparently expected to more than double the number of diesels running in and out of Union Station, and the local politics is apparently starting to petition that the line be electrified right from the start.
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User: furrfu
Date: 2009-12-23 16:23 (UTC)
Subject: (no subject)
Yes, I was once quite surprised at King's Cross to see just how much soot was being belched out by a Great Northern diesel setting off. The lack of local pollution (and noise -- although that's just another kind of polluation) is obviously a big plus for electrics.

And more efficient, too, even if the electricity was generated from burning oil (because most—not all—diesel trains are, essentially, electrics with a big diesel generator).

I don't know if any trains (ie not metro systems or trams) use regenerative braking yet. I imagine it wouldn't be worth the trouble on long-distance high-speed trains, but might be worth it on commute trains.
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The Bellinghman
User: bellinghman
Date: 2009-12-23 16:31 (UTC)
Subject: (no subject)
I'd be interested to see the comparative efficiency, to be honest.

Yes, the electrics don't have to lug along a generator and fuel tanks. And yes, the power stations get economy of scale that a mobile generator can't achieve.

On the other hand, long miles of cabling can lose quite a bit of juice due to cable resistance.
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User: furrfu
Date: 2009-12-23 16:51 (UTC)
Subject: (no subject)
Sadly, I have found only partial information so far:

- Wikipedia says transmission losses in the US were estimated at 7.2% in 1995
- Wikipedia says fossil fuel power plants with steam turbines can be up to 60% efficient
- A very modern piston diesel engine can be 35% efficient. Googling for train diesel engine efficiencies seems to suggest 30%.

Sadly, I lack one piece of the puzzle, and that's an estimate of the losses in the overhead wire. The UK operates (according, again, to Wikipedia) at 25kV AC, but I can't find exact numbers for efficiencies just now. However, I doubt it's enough to make up the difference.
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Tony Finch
User: fanf
Date: 2009-12-23 17:08 (UTC)
Subject: (no subject)
I believe TGVs use regenerative braking. Trains can't rely on regenerative brakes, though, because there must be some other load on the line into which the slowing train can pump its power. If that isn't the case, trains use rheostatic braking instead (or some combination of the two if the line can take part of the braking load). TGVs have stacks of resistors and cooling fans on top of the power cars for this purpose.

TGVs only use their disc brakes below a certain speed.
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User: furrfu
Date: 2009-12-23 17:13 (UTC)
Subject: (no subject)
Yes, indeed. I should have phrased it better.

"how many trains use regenerative braking with the express purpose of feeding energy back into the grid."

(I know the Brussels Metro switched to such a system not that long ago, and became quite a bit more efficient because of it. But that's not big trains.)

(Incidentally, huge dump trucks as used in open cast mining also use resistive braking. Because disc brakes just can't cope.)
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Tony Finch
User: fanf
Date: 2009-12-23 17:19 (UTC)
Subject: (no subject)
Ah right, I see what you mean. The need for rheostatic brakes rather implies that there's no feedback to the general grid.

Speaking of brakes not coping, the French have been investigating magnetic brakes for very high speed braking, which work by transferring energy to the rails by heating them up using induced eddy currents. You can't use this technique at lower speeds because you don't want to heat the rails up too much, i.e. you want to spread the heat out as much as possible.
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Jos Dingjan
User: happydisciple
Date: 2009-12-23 21:58 (UTC)
Subject: (no subject)
I don't know if any trains (ie not metro systems or trams) use regenerative braking yet. I imagine it wouldn't be worth the trouble on long-distance high-speed trains, but might be worth it on commute trains.
The Dutch Railways' DD-IRM EMUs use regenerative braking (not mentioned in the English article, but it is in the Dutch one), as did the short-lived SM '90.
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The Bellinghman
User: bellinghman
Date: 2009-12-23 18:27 (UTC)
Subject: (no subject)
Oh, one other point about the Canadian trains: they're apparently considering the possibility of electrification of the line where it goes over the Rockies. Not the whole line, just the mountain bit.

The vast majority of that traffic is freight, with two, sometimes three, great diesel-electric haulage units on the front.
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User: furrfu
Date: 2009-12-23 18:31 (UTC)
Subject: (no subject)
Yes, according to Wikipedia — which seems to be very thorough when it comes to anything to do with trains :-) — electric is more powerful than diesel is more powerful than steam. Similarly more efficient, too.
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