Why do DC trains have pickup shoes for the 3rd and 4th rail, rather than wheels?

[Markuk]

Simple question as title! Surely to the uneducated like me, wheels would be more reliable and less prone to causing infrastructure issues as can happen when pickup shoes fail.

If it's about contact area, then surely it's a case of more wheels (just like the running wheels are the current return on 3rd rail lines).

 

[twpsaesneg]

Simple question as title! Surely to the uneducated like me, wheels would be more reliable and less prone to causing infrastructure issues as can happen when pickup shoes fail.

If it's about contact area, then surely it's a case of more wheels (just like the running wheels are the current return on 3rd rail lines).

The con rail is discontinuous so whilst you could use a wheel it would need to go on and off the rail in the same way that shoes do, and it would be liable to being damaged in the same way that shoes are, but with a more complex and expensive bit of kit than a block of carbon.

 

[The exile]

The con rail is discontinuous so whilst you could use a wheel it would need to go on and off the rail in the same way that shoes do, and it would be liable to being damaged in the same way that shoes are, but with a more complex and expensive bit of kit than a block of carbon.

If you had a wheel /axle you would still need a way of getting the electricity from the wheel to the motors (a wheel has to rotate and thus can’t have fixed connections - this one would also have to be insulated from the rest of the train). Collector shoe on insulating bar has stood the test of time….

 

[stuu]

I suspect the current would weld any bearing solid pretty quickly. There must be a good reason third rail electrification uses shoes everywhere

 

[norbitonflyer]

Some early trolleybuses and tram system use trollley wheels rather than sliding contacts (hence the name)

 

[furnessvale]

I suspect the current would weld any bearing solid pretty quickly. There must be a good reason third rail electrification uses shoes everywhere

The hefty currents of 25Kv return through the loco wheels.

 

[swt_passenger]

The hefty currents of 25Kv return through the loco wheels.

All stock, whether AC or DC, has additional axle end mounted brushgear on nearly all the axles to allow the return traction current to bypass the wheel bearings.

 

[aleggatta]

Shoes have a higher contact surface area, return current still goes down through running wheels, but even for such large wheels the surface area on the return side is still only that of a 10p piece, and it only really works as there are multiple wheels in contact with the running rails.

If you were to swap for wheels, you wouldn’t get the surface area as there isn’t simply the space for them to be large enough to create a large enough surface area, and small enough to fit between all the gubbins.

Add to that, if the carbon brushes on this 3rd rail wheel were to be defective, the bearing would just Weld itself together, and you would have a dragging wheel.

The current system works, with softer metals on the shoegear than the rails, meaning things generally last quite well

 

[furnessvale]

All stock, whether AC or DC, has additional axle end mounted brushgear on nearly all the axles to allow the return traction current to bypass the wheel bearings.

Yes. I was really referencing the difference, or lack of it, being a reason for a 4th rail.

 

[Crithylum]

The hefty currents of 25Kv return through the loco wheels.

The current of a 25kv system is 33x less than with 750V DC (for same power). (I = p/V)


The immense weight of the train (on the running rails) means that the theoretically inifinetly small contact point is instead the size of a coin, which is small for something bearing that much load, but is quite a large cross section for a conductor. A 3rd rail wheel will not have this force to maintain a large contact patch, so will therefore have a smaller area (and higher resistance).

On a tangential note, how viable would it be to have 1 running rail +ve, and the other -ve, or the ac equivalent? This is what model trains use.

 

[renegademaster]

On a tangential note, how viable would it be to have 1 running rail +ve, and the other -ve, or the ac equivalent? This is what model trains use

You couldnt have a joined up axle, youd need electrically independent wheels, its been done with trams but its an extra pain to implement for no real gain

== Doublepost prevention - post automatically merged: 15 Oct 2024 ==

The hefty currents of 25Kv return through the loco wheels.

3rd rail current returns through the running rail.

Underground is different in that you have 4 rails, 1 + 1- , where the running rails shouldnt be able to form an easy return path

 

[furnessvale]

The current of a 25kv system is 33x less than with 750V DC (for same power). (I = p/V)


The immense weight of the train (on the running rails) means that the theoretically inifinetly small contact point is instead the size of a coin, which is small for something bearing that much load, but is quite a large cross section for a conductor. A 3rd rail wheel will not have this force to maintain a large contact patch, so will therefore have a smaller area (and higher resistance).

On a tangential note, how viable would it be to have 1 running rail +ve, and the other -ve, or the ac equivalent? This is what model trains use.

A modern 25kv freight loco puts everything back through 4 axles compared to the spread of shoes along an EMU.

Love the last idea with kids putting a bar across both rails, or attempted suicides laying across the tracks not having to wait for the next train!

 

[Crithylum]

Love the last idea with kids putting a bar across both rails, or attempted suicides laying across the tracks not having to wait for the next train!

Isn't that already the case with 3rd/4th rail?

 

[edwin_m]

There wouldn't be room within clearances for extra wheels, bearings etc above the third rail.

Putting the voltage across the two running rails would mean every train using the route, electric or otherwise, would need insulated wheelsets. This is very difficult to do reliably, considering the sorts of stresses a wheelset experiences.

 

[cool110]

On a tangential note, how viable would it be to have 1 running rail +ve, and the other -ve, or the ac equivalent? This is what model trains use.

Have you ever tried to make a balloon loop on one of those? You need some complex switching arrangements to avoid everything shorting out, get similar problems with triangles.

 

[Merle Haggard]

All stock, whether AC or DC, has additional axle end mounted brushgear on nearly all the axles to allow the return traction current to bypass the wheel bearings.

When the Euston - Watford D.C. line was converted from 4 rail to 3 rail this arrangement wasn't used - on its way to the rail, the return was going through the bearings. What could go wrong?
About 1970 I was tangentially involved in a panic because exams showed serious damage to the bearings and axles.
You might have expected engineers to have heard about 'spark erosion', it is used in metalworking

 

[furnessvale]

Isn't that already the case with 3rd/4th rail?

Contact with the 3rd rail, especially in dry weather, can often not be fatal.

I had assumed you we talking about replacing 25kv OHLE with using the running rails, I can see now you were probably just eliminating the 3rd rail!

 

[LYradial]

It woul solve the leaf fall problem tho’ as you would not be able to move in the first place, shoes at least are self cleaning to a degree

 

[plugwash]

On a tangential note, how viable would it be to have 1 running rail +ve, and the other -ve, or the ac equivalent? This is what model trains use.

I see several difficulties.

1. It would be incompatible with all existing rolling stock, rolling stock would have to be specially designed with insulating axles.
2. It would make it much harder to move the danger around. A "third rail" can switch sides to keep it away from platforms and can be interrupted on level crossings.
3. It would preclude use of "track circuits" for signalling
4. It would make it impractical to ground the vehicle bodies, since even if one rail was grounded trains can get spinned round. This would make it very difficult to make the system "single fault safe"
5. It would create complications around pointwork and particularly triangles.

Most of these are much less of an issue at model scale, because models can operate off "safe" voltages.

 

[edwin_m]

Have you ever tried to make a balloon loop on one of those? You need some complex switching arrangements to avoid everything shorting out, get similar problems with triangles.

I'm looking forward to Network Rail installing a giant DCC reversing loop module.

 

[Prime586]

When the Euston - Watford D.C. line was converted from 4 rail to 3 rail this arrangement wasn't used - on its way to the rail, the return was going through the bearings. What could go wrong?
About 1970 I was tangentially involved in a panic because exams showed serious damage to the bearings and axles.
You might have expected engineers to have heard about 'spark erosion', it is used in metalworking

The same thing happened to the Merseyrail 50x fleet in the summer of 2022 after the line voltage was jacked up for the 777s and Stadler took over the maintenance. There were over 300 recorded bearing failures in just over a month, which led to stock shortages and reduced service as they ran out of replacement wheel bearings and had to investigae what the cause was (IIRC Merseytravel never made public the final report from the investigation).

 

[BayPaul]

I believe that 3rd rail material is optimised for conductivity, while normal rsils are optimised for strength, so presumably you'd lose a lot more energy to resistance in the rails.

Also, the main rails have a lot more and more substantial supports. It would be very difficult and expensive to make these all insulators.

 

[satisnek]

I see several difficulties.

1. It would be incompatible with all existing rolling stock, rolling stock would have to be specially designed with insulating axles.
2. It would make it much harder to move the danger around. A "third rail" can switch sides to keep it away from platforms and can be interrupted on level crossings.
3. It would preclude use of "track circuits" for signalling
4. It would make it impractical to ground the vehicle bodies, since even if one rail was grounded trains can get spinned round. This would make it very difficult to make the system "single fault safe"
5. It would create complications around pointwork and particularly triangles.

6. There are no giant-sized fingers to push the train off dead spots.

 

[millemille]

I believe the OP's question has got nothing to do with power transmission through the running rails, they are asking why current collecting shoes ("slippers") with a sliding contact with the conductor rail are used instead of a rotating wheel in contact with the conductor rail?

The immediate answer that springs to mind is the current collecting wheel would have to be so large, to avoid the bearing upon which the wheel rotated spinning too quickly, that it would be completely impractical. That's before you start talking about conductor cross sectional area, impedance, electrical erosion and so on.

 

[swt_passenger]

I believe the OP's question has got nothing to do with power transmission through the running rails, they are asking why current collecting shoes ("slippers") with a sliding contact with the conductor rail are used instead of a rotating wheel in contact with the conductor rail?

I think some recent posters are answering the suggestion in post #10, which went off at a tangent…