Most of them have substantial above-ground portions, though. The London Underground, for example, has 55% of its track above ground. But yes, snow can cause problems.
Which is why Docklands has gone for a power rail with underside contact.
Edit: well, one reason. Also safety to people on the line.
Yes, weather can cause problems but on the other hand I can attest to the fact that the Southern Region of British Rail has been running a very reliable service for the last 70-odd years using third rail electrification. And as I said deaths involving the third rail are uncommon, even with wild animals. (Southern Region uses an overhead system for freight yards as the trains are equipped to use both systems.)
Yup (on all counts) - the human fatalities we get on this part of the rail network are almost all due to ‘person hit by a train’ - it’s difficult for people to stray onto the track except on purpose - tracks are all fenced off, platform ends are gated or hobbled (as are the road edges of level crossings.
In the event that a person does get on the track (and is noticed), the power is cut until the incident is resolved (to the great disruption of commuters).
The snow we had winter before last played havoc with the electrified rails, just because it was settling on the rail and freezing - there’s a blower ahead of the pickup shoe on some trains, but it wasn’t sufficient to clear the snow and so the contact was intermittent.
There’s one very fundamental reason why the two rails are not used for power, you’ll need split axles for that, and a train with split axles doesn’t work.
The wheels on trains are slightly conical, and paired together, so that if for example, the train is negotiating a curve, the wheels tendo to move outwards, as the wheels move out the outer wheel presents a larger diameter to the rail than the inner wheel, since they are paired together they can’t turn at different speeds, thus the result is a force that tends to rotate the wheel pairs back to the center of the track. That’s what keep trains on track.
Any middleschool physical science textbook should do the trick if you can’t remember back that far.
You’d have to insulate the whole rail system, even when it rains or when snow is piling up. Good luck with that.
Okay, forget powering the trains (apparently that’s a tangent-laden minefield). WHat about drawing power from the trains when their braking, rather than letting that go to waste?
In the NYC subway, all models of subway car since the R142 have regenerative braking. Energy is fed back into the third rail when slowing down.
They don’t always work that well, though.
I would take that as obvious common sense (maybe combined with an engineering background? hard to factor that out of my own view). A fluid or electrical transmission system is obviously going to use energy heating up the fluid (from friction as it flows) or the generator/motor coils, but also have various bearings (in which the prime move/generator, transmission and wheels turn) where a small amount of energy is lost. The solid mechanical drive has only the latter.
In Europe hydraulic transmission diesel locomotives are common along with electric, though electric has tended to gain ground in recent decades. In North America, diesel hydraulic locomotives are unheard of, outside very small special ones like mine locomotives. Some smaller diesel hydraulic locomotives have a ‘lock up’ feature in the transmission, like a car’s automatic transmission, where the drive is solid mechanical at higher speed.
Also as mentioned, very high voltage (and thus most efficient) central electric power for trains is only practical with overhead catenary. A third rail, let alone having one of the two rails transit power, is limited by insulating the rail from the earth at high voltage, plus the safety issue.
Catenary electric is common on even freight relatively long distance railways in many parts of the world, and was not unknown on North American freight railways as steam was replaced, though diesel was much more common. However electric loco’s later disappeared from NA freight railroads. It’s a matter of the comparative economics of building and maintaining all the overhead lines, as opposed to the potential cost savings in generating the electricity. A central power plant won’t necessarily exceed a modern diesel in thermal efficiency, and there are transmission losses. But, the central station might burn a much cheaper fuel (coal v diesel fuel), or have environmental benefits (zero CO2 nuclear or renewables v burning diesel fuel), or it might be some combination of more thermally efficient and lower CO2 (large combined cycle gas turbine plant v a locomotive’s diesel). Also, electrified trains can easily be fitted to use their motors for braking on down grades and feed the power back into the grid. Normal diesel loco’s use resistors to employ their motors as brakes, turning the downhill rolling energy to heat. Lately some manufacturers have introduced diesel loco’s with high capacity battery banks to store braking power, but again with electric trains that feature comes pretty much automatically.
The reason diesel is totally dominant in North American freight railroding is a combination of relatively low density of rail line use (whereas the cost per mile to put up and maintain catenary is fixed), the same lack of policies to push up end use oil prices, and lately specifically to lower CO2 emissions, that cause a lot of the other differences in transport solutions in North America v Europe, and perhaps most fundamentally the fact that European rail networks were set up as govt run entities with secondary emphasis on economic efficiency, which still carries over even though operations have in some cases been privatized since.
Regenerative braking. Yup, they do that too. You still need normal brakes, too, since the effectiveness of regenerative braking depends on the speeds, so you can never actually stop something entirely with them. So you use the regenerative brakes to soak up most of the energy, and then put on the wasteful friction brakes to get rid of the last little bit.
Diesel engines run generators that run electric motors that drive the wheels.
Regenerative breaking decouples the generators from the diesel engines and causes the electric motors (turning from the wheels only now) to run the generators in reverse. This slows the train down. During breaking, the generators produce electrical energy that heats up very big ass resistors. This energy is “wasted” in heat, but the train is able to stop after bringing coal from halfway across the country.
What an incredibly useless contribution.
Look, it may very well be that a mechanical coupling is more efficient than an IC/electric combination but it would depend on the efficiency of the components involved. It may or may not be the case depending upon the complexity of the transmission, which in the case of a locomotive would be extremely complex with lots of bearings and friction between gears Vs. the efficiency of the generators and motors.
But snarky comments are not cites.
This is very true, but there is also a more pressing safety reason to do this. The Westinghouse air brake (extra tech has been added to it, but the basics are unchanged for about 140 years, as it’s a good, mature technology) is fail safe in most cases, but it is NOT idiot-proof. If you waste the air by making little and frequent applications on the Westinghouse, just to slow the train, then you might find you are getting dangerously low on air when you really need it. At best, this means you will have to come to a complete stop, even when you don’t want to (cue cranky calls from the signalling guys asking why you’re blocking the main line). You need to wait for the air to recharge. At worst, a runaway. Also, the dynamic brakes are infinitely variable in either the stronger or weaker direction (like the brakes on your car - if you apply too much or too little, you can always correct). Air brakes (depending on their level of technology - often simpler in freight trains than passenger ones) don’t have this option, necessarily. You might not be able to lessen the braking effort without first completely releasing the air brakes, and then applying a lighter touch.
The upshot of all this is they use the “Dyno” whenever they can, but sometimes have no choice but to apply some air. In my neck of the woods, this usually happens on 5000 ton coal trains negotiating a 1 in 33 descent down a mountain pass. They have no choice but to use some air sometimes, but they try to avoid it.
San Bernadino Runaway - some interesting stuff here for railway geeks or anyone with an interest in this. Explains the limits of dynamic braking, Westinghouse, and mismanagement.
Click link for full article
Going back to the original question, as I assume it was meant. That is the difference of having the power plant at the diesel locomotive, verses using central power, and which is more efficient.
Now take that question to long haul freight rail lines, say 1500 miles.
Electricity does not run for free, you lose some of it in line loss. Wouldn’t the line loss be a significant for a long rail line like this ?
Also isn’t the traction power from DC current, and I was under the impression that DC current did not travel long distances well ?
The line loss depends on how far you are from the power station (along with other variables, like the voltage on the wires between the power station and the train). On line as long as 1500 miles, you’re going to be drawing power from several/many power stations along the way: the overhead wires won’t just be connected to the power grid at one point.
For example, in Japan, the Tōkaidō, Sanyō and Kyushu Shinkansen, from Tokyo to Kagoshima-Chūō via Shin-Osaka and Hakata, is a total of 1326 km of electrified track operating as one railway line. I don’t know the details, but a line that long would be connected to the power grid at multiple ponts
Note also that a steam cylinder can generate torque from a standing start, so can be coupled directly to the driving wheels. You can’t do that with a diesel, which is why you need a clutch and gearbox (well, a clutch for sure; a gearbox, probably, for the same reason why you need it for a car).
That’s interesting (honestly - I didn’t quite realise the subtlety of it) - but I imagine a solution could be engineered nowadays to keep the wheels mechanically coupled, but electrically isolated - something involving ceramics or composite materials - but it’s probably just not worth trying.
Another problem would be ground isolation of the hot rail - the third rail here is not load-bearing and thus can be easily supported on ceramic and plastic insulators (and raised above the level of the running rails) - a live running rail would probably require a rethink of the whole track construction system.