Because the forces on the transmission would be enormous and varying as you need to change gear, whereas electric motors don’t need a gearing system where they can change ratios because they produce all their torque whenever, making it pretty infeasible
A direct mechanical connection, as with an ordinary gearbox & clutch is the most efficient form of power transmission. Any time an intermediate connection such as a fluid drive or electrical transmission is added to the system there is some corresponding loss of power. The problem with a direct drive is that it becomes unmanageable beyond a certain size/horsepower level. I’ve heard that with an internal combustion engine about 300 hp is the upper limit, but that sounds a little low to me. Modern locomotivey s crank out about 5000 hp or more, obviously well over the limit. Additionally, one might consider
the difficulty of shifting gears underway with the momentum of 100 or more rail cars exerting their influence, while avoiding jerks and stalls that can break a train in two. Somewhat more complicated than punching the clutch and shifting in a typical automobile.
That said, there have been locomotives built with direct-drive transmissions. Here is a picture of a small vintage locomotive that I had a hand in restoring. It is equipped with a large-displacement gasoline engine of about 150 hp, and a clutch so massive it is operated with a long hand lever instead of the more common foot pedal.
The question then becomes: if it’s so advantageous, then why weren’t steam locomotives built that way? The answer is mostly that the technology wasn’t available at the time, but in fact some steam-electric locomotives were built. They seemed to work reasonably well but were probably technology before their time.
Steam-turbine-electric locomotives were also a thing.
Short answer is longevity. This design allows an 8520cid/1392L two-stroke diesel engine to run at its 900 rpm redline for a VERY long time. How many gears would a mechanical transmission require and how big would the clutch or torque converter have to be? How long would that last?
Check out this howstuffworks link.
The overhead wires only form the live conductor. One side of the circuit. The other side is the earth return…
The rails form the earth return, so they are electrically connected together at any gap, and also well earthed into the wet earth. The rails are properly earthed to avoid stray currents flowing across the surface (the surface is conductive due to biological substances…) and then descending into the wet depths via the poles… If the earth return flows via the poles, the poles corrode faster due to electrolysis effects.
One problem suffered by this system is that in autumn leaves on the line can cause trains to lose power and is a source of delays and frustration familiar to many London commuter.
It should be noted that the third rail is suspended on ceramic insulators. These would not be able to bear the enormous stresses experienced by the main track.
:dubious: The one I quoted…
This is your opinion or do you have a cite?
I believe I can explain: trains are efficient in cost. They are not efficient in time. Rail actually takes at least as long, and usually much longer, than a comparable truck route.
They’re also horribly inflexible, which is one reason they’ve been slowly driven out of the passenger market in the US. Heavy rail is a lot more efficient for the profitable commercial market, where the passenger market was always a very marginal business.
Once the truck has been stuck onto a rail, you ain’t gonna touch it until the railway lets it off. You cannot direct it, track it, and if it’s late… then tough for you. This is especially worrisome if you need your delivery planned very precisely, and nearly everybody does just that, or have perishables. Which isn’t to say rail isn’t useful - it’s extremely useful for bulk freight when you can plan in advance. It’s dead-last in flexibility to everything except river shipping, however, and that’s a tradeoff which is very hard to make in a modern economy. Logistics in the 21st century relies on quick, often last-minute decision making,
As far as the power-economy, I would note that the power-plant is going to have to electrify the entire rail system, not one locomotive. That’s going to be big, big energy losses just from the incidental inefficiencies, particularly when you’re talking about nation-spanning rail grids. Upgrades would require big investments of time and equipment across a broad swath of the network and require simultaneous rollout , whereas you can now just buy new engines. Meanwhile, it’s also going to require much more, and much more careful, maintenance and a lot more people to fix it. It’s no accident that third rail electrification is almost exclusively used in relatively dense areas which already have large support staffs in close proximity, and often closed environments.
Call it an opinion if you like, as I don’t care to go looking for a cite. But I don’t think you can rebut it, at least as it applies to this subject. The only other power transmission systems used in railroad locomotives - with the exception of straight electrics, are internal combustion-hydraulic and IC-electric. Both are subject to parasitic power consumption by either the generator or the hydraulic pump. A hydraulic drive also loses power to the slippage inherent in any fluid link. A direct mechanical link delivers nearly all the output horsepower to the drivers, with only a tiny power loss to friction from bearings and gears.
SS
In what way does that support the premise?
Direct mechanical drive is really only found in the tiniest shunting units and lightweight railcars, and even then, it’s a thing of the past. These days, they use a hydraulic transmission: no gearbox as such, just two (usually) or maybe more torque converters similar to the ones in automatic cars, each driving different ratios of fixed gears. So usually just two settings for the driver to choose from. Only a small handful of places (Japan and Germany, from memory) have used this tech on larger locomotives, but most railways shun it in that role.
I do suspect trying to start a 5000 ton coal train moving would be instant fried clutch, but assuming it were possible, there is a further issue - heavy trains use multiple locomotives, all controlled by one guy in the lead unit. Anybody who’s driven manual cars will know that every clutch is slightly different as far as the friction point goes, but you’d need some way of having servo-motors operate all the clutches such that they slip to the same degree at the same moment. Suspect that’d be no easy task.
Slight, nerdy, railfan nitpick here (sorry)… 
The London Tube system is actually a rare FOUR rail one. The traction current uses two rails specific to that purpose, and the running rails are not part of the circuit. This is to keep the entire circuit supported on insulators, which prevents stray current from corroding the tube itself (older sections are often metal) as well as building foundations.
[QUOTE=The Second Stone]
Is that catenary wire copper, or can it be aluminum?
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Trolley or contact wire (the straight wire that the pantograph contacts) is usually copper because in a given diameter, itstensile strength is twice or more as aluminum. The catenary could be aluminum except for that pesky problem of needing to make thousands of technically challenging and expensive mixed-metal connections. (Aluminum vs copper connections are the primary reason aluminum wiring has a somewhat undeserved reputation for causing house fires.)
Am I wrong in thinking that in a diesel-electric setup, the electric motors ARE the transmission for the diesel engine?
Bit of a semantic quibble, but I think it’s more commonly regarded that the entire electric drive system is the transmission, but if you’re going to single out one particular component, it would be the generator more than the motors, but really, it’s the whole electric traction system.
Interesting distinction.
My initial reaction was to think no. But then I thought, if somebody asked me how a diesel locomotive applies power to its wheels, I might just say, “electric transmission”. So yeah.
On the other hand, a diesel loco is just an electric one that happens to carry its own power station around with it. You probably wouldn’t say that a pure electric loco uses electric transmission for its remote coal or nuclear power source. And for the train crew, they are very much interested in what the traction motors are doing, moment to moment. Not that they don’t monitor the diesel prime mover, but it tends to just play along with whatever the power demands are at any given time, and so isn’t front of mind for the driver so much. If that makes a difference.
Seems to all depend on context.
Maybe I’m thinking: if there is direct kinetic/mechanical energy (steam, diesel-hydraulic etc), then you have a transmission, but if there is a break where it’s in another form of energy, then not.
Also, 3rd (or 4th) rail systes are most common in underground subway lines, where they are mostly protected from weather. Growing up in western Minnesota, I have seen times after winter storms when the tracks are covered with snowdrifts so high that the railcars can’t be seen, They have huge rotary snowthrowers to clear off the tracks after such storms. Can you imagine the electrical losses or shorts from a high-voltage 3rd rail when it is buried under 8 feet of wet snow? It would be a real trick to keep it operating when exposed to a Minnesota winter. Or even a heavy summer thunderstorm.