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Why are there no single or two-car EMUs?

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hexagon789

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Because all EMU's I know of have unpowered coaches-
323 (1 out of 3)
319 (3 out of 4!)
331/0 (1 out of 3)
331/1 (2 out of 4)

But in that case if you couple two 319s you have eight cars with double the power of a single unit. If you have a four coach loco-hauled train, making it up to eight coaches unless you add another loco, you've no more power to compensate for the extra load.
 

norbitonflyer

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Because all EMU's I know of have unpowered coaches-

Flipping through the 2019 stock book, I find classes 710, 730, 777, 399, and the Underground's S stock and 1992 stock as counterexamples with all cars powered. The installed power in an emu is chosen to be proportional to the number of cars so, for example, a 2HAP had one power car with one powered bogie, a 4CEP had two power cars each with one power bogie, and a 4CIG had one power car with two power bogies, so all units had one powered axle per car, and all had similar performance. (Modern comparisons are more complicaterd, because there is no longer any standardisation of traction equipment)
 

Deepgreen

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They're still EMUs, just like Class 153 is still a DMU. The M signifies that it can work in multiple with other units — not that the fixed formation is comprised of multiple carriages.
Indeed - "ESU" was light-hearted. It's 'comprised' or 'composed of', not "comprised of...".
 

gingertom

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It turns out the PKP have a somewhat modern ESU, the EN81 from 2005. Not the most powerful, still interesting. (Mostly used on long-ago-electrified now not very highly used lines from what I remember.)

Stadler do build 2-car Flirts, but they aren't fully motored (most likely 1 axle powered per motor bogie, I can't find the datasheets).
IIRC it is one "normal" bogie fully motored and the rest of them unpowered. But that would have been spec'ed by the customer, if a 2-car with 2 fully powered is asked for that's what they get. If you want to be totally silly then dispense with Jacobs bogies and have a 2 car set with all 4 normal bogies motored- 5200bhp = good for 200mph. It won't ever happen but I reckon it would be quite quick off the mark.
 

Deepgreen

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stopping it would be more fun
Power and braking are in generally in proportion to number of cars, so a single car (assuming proportionate power) would accelerate and decelerate much the same as multiple cars. In fact, several identical units in multiple are marginally faster than a single, because they present the same air resistance from the front, but the units coupled behind the leading one don't experience that resistance. Only becomes sensibly measurable at 90+mph really.
 

edwin_m

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Power and braking are in generally in proportion to number of cars, so a single car (assuming proportionate power) would accelerate and decelerate much the same as multiple cars. In fact, several identical units in multiple are marginally faster than a single, because they present the same air resistance from the front, but the units coupled behind the leading one don't experience that resistance. Only becomes sensibly measurable at 90+mph really.
However braking has to allow for the possibility of part of the system being isolated, which would be a minimum of 25% on a single car. Also a longer unit may get better adhesion on average, because the leading wheels clean the rail for the later ones.
 

Deepgreen

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Yes I'd forgotten about the South Acton shuttle, but it's definitely not the only example, as I pointed out above in my post #71. The Waterloo&City used to run a single car off-peak. I'm guessing the W&C was the only example on BR, unless you know different.
I think our posts 'crossed', or I just missed it. Yes the 'Drain' was often singles.
 

Deepgreen

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However braking has to allow for the possibility of part of the system being isolated, which would be a minimum of 25% on a single car. Also a longer unit may get better adhesion on average, because the leading wheels clean the rail for the later ones.
Yes, although I've always debated the rail cleaning idea - the leading wheels often just crush the leaves and spread the goo evenly over the rail head for the following ones to encounter.

It would be interesting to know how the MLVs fared in terms of brake power. For those who've never seen one operating alone, this is one (9009) I captured approaching Horsham in the 1980s.
 

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edwin_m

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Yes, although I've always debated the rail cleaning idea - the leading wheels often just crush the leaves and spread the goo evenly over the rail head for the following ones to encounter.

It would be interesting to know how the MLVs fared in terms of brake power. For those who've never seen one operating alone, this is one (9009) I captured approaching Horsham in the 1980s.
I used to work with some of the experts at BR Research who seemed pretty convinced the wheel cleaning effect existed. The MLV would have tread breaks, as incidentally does a 153, which are not as good in general as discs but are better at cleaning leaf residue off the wheels.
 

Deepgreen

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I used to work with some of the experts at BR Research who seemed pretty convinced the wheel cleaning effect existed. The MLV would have tread breaks, as incidentally does a 153, which are not as good in general as discs but are better at cleaning leaf residue off the wheels.
Certainly much more likely with treads than discs.
 

Jozhua

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I know in the US married pair EMU's are quite popular. That is, in the places that are lucky to have rail infrastructure. I believe the MTA and CTA's heavy rail MU fleet mostly consists of married pairs, this is both on their respective Subway and commuter rail systems.

MLV single vehicle 500hp should be faster than a 1000hp 4 car vep Brighton to London fast? Nope; a miserable 75-80mph at a push, compared to a VEP where 90+mph at 2 or 3 points

Why? Because the MLV has a cam shaft, so every time you lose power it takes 10-15 seconds to get back to full power. There are many gapes in the power supply, as many as 40 between London and Brighton, with every set of points, foot crossings, badger crossings etc, giving a great opportunity to save energy:frown:

I'm presuming MLV means 3rd rail and VEP means overhead or diesel/self power?
 

43096

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I know in the US married pair EMU's are quite popular. That is, in the places that are lucky to have rail infrastructure. I believe the MTA and CTA's heavy rail MU fleet mostly consists of married pairs, this is both on their respective Subway and commuter rail systems.



I'm presuming MLV means 3rd rail and VEP means overhead or diesel/self power?
No, types of third rail stock. MLV = Class 419, VEP = Class 423
 

jopsuk

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MLV: motor luggage van
VEP: properly 4VEP, aka Vestibuled Electro-Pneumatic Brake, a 4-car suburban unit type built for suburban work in the 60s.

Up until the late 80s it was the norm for British 4-car EMUs to be one motor coach and three unpowered. The high-power 4-REPs had two motor coaches but were built to push or pull one or two addtional 4-car unpowered units. The 70s-designed Class 313-315 & 507-508 were a new way of thinking with two motor coaches, even in the 3-car ones, but the 80s Mark 3 based units reverted to single motor coaches, even with the 5-car, 23m coach, Class 442. The Networkers went back to two motor coaches, but Electrostars have three (or four in a five car unit) but without all axles in all motor coaches being powered. Siemens for 4-car Desiros however have stuck with 2 motor 2 unpowered
 

rebmcr

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Yes, although I've always debated the rail cleaning idea - the leading wheels often just crush the leaves and spread the goo evenly over the rail head for the following ones to encounter.

It would be interesting to know how the MLVs fared in terms of brake power. For those who've never seen one operating alone, this is one (9009) I captured approaching Horsham in the 1980s.

I used to work with some of the experts at BR Research who seemed pretty convinced the wheel cleaning effect existed. The MLV would have tread breaks, as incidentally does a 153, which are not as good in general as discs but are better at cleaning leaf residue off the wheels.

I recall from a documentary that Pacer drivers are — were? — trained to use a braking technique (full service, no 'ramp up' graduation) that takes advantage of the wheel cleaning effect.

I don't doubt that such an effect is negligible for acceleration, rather than braking.
 

jopsuk

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meanwhile in the US if the Pennsylvania Railroad had had its way the Budd Metroliner would have been built entirely as single car units. The DOT wanted 4-car units, the compromise was 2-car. And not some 2-bit suburban unit, 150mph (design- 120mph in actual service, 164mph in testing) intercity vehicles
 

James James

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They're still EMUs, just like Class 153 is still a DMU. The M signifies that it can work in multiple with other units — not that the fixed formation is comprised of multiple carriages.
The ESU terminology was for fun :) - but:
- On what authority do you stake your claim? Wikipedia claims a single car is "generally" an EMU, but that implies they sometimes aren't. The OED's definition explicitly specifies "An electric passenger train of two or more carriages" (https://www.lexico.com/definition/electric_multiple_unit).
- I've never seen an EN81 operate in multiple.
 

rebmcr

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The ESU terminology was for fun :) - but:
- On what authority do you stake your claim? Wikipedia claims a single car is "generally" an EMU, but that implies they sometimes aren't. The OED's definition explicitly specifies "An electric passenger train of two or more carriages" (https://www.lexico.com/definition/electric_multiple_unit).
- I've never seen an EN81 operate in multiple.

The systems like Blue Square installed on locomotives and single-carriage designs like Class 121 were already called Multiple Working.

I wouldn't place much faith in the ability of English Language professors (i.e. the writers of the OED) to understand technical specifications with 100% accuracy.
 

Snow1964

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The original London Brighton and South Coast Railway (overhead) electrics were motor luggage vans. They were single vehicles (with cabs) formed in middle of 5 car trains (driving trailer+ trailer either side). They had guards and luggage compartments

The LSWR converted some vehicles into 2car trailer sets. Used to strengthen some peak hour trains to 8 car, former between pair of 3car electrics. Years later sets were reformed as 4 car EMUs

Both are examples of 2car trailer sets designed to run with higher power electric vehicles
 

AM9

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It seems that most of the discussion here is off-topic about 3rd rail two-car EMUs rather than the 25kV types that the OP said the thread was abou in the first post. Since 25kV was established in the UK, to my knowledge, there has only ever been one two-car type, i.e. the class 309/1. Two-car 3rd/4th rail EMUs are not that uncommon as types.
 

norbitonflyer

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Indeed, the only overhead electric 2-car types I know of other than Class 309 are the LBSCR's South London units on 6600V ac (originally built in 1908 as 3-car but converted to 2-car in 1910) and the two units built for the short-lived (1913-18) experimental Bury - Holcombe Brook 3500V dc scheme on the Lancashire & Yorkshire Railway. Both lines (and their trains) were later converted to 3rd rail.

Most ac units have only one heavy transformer and the other extra gubbins an ac unit needs, so it seems to make sense to minimise the number of these in a long train. Therefore an ac train made up of two 2-car units would be much more complicated than one made up of a single 4-car unit, whereas on dc there is little difference between the two formations other than the redundant cabs
 
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AM9

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Indeed, the only overhead electric 2-car types I know of other than Class 309 are the LBSCR's South London units on 6600V ac (originally built in 1908 as 3-car but converted to 2-car in 1910) and the two units built for the short-lived (1913-18) experimental Bury - Holcombe Brook 3500V dc scheme on the Lancashire & Yorkshire Railway. Both lines (and their trains) were later converted to 3rd rail.

Most ac units have only one heavy transformer and the other extra gubbins an ac unit needs, so it seems to make sense to minimise the number of these in a long train. Therefore an ac train made up of two 2-car units would be much more complicated than one made up of a single 4-car unit, whereas on dc there is little difference between the two formations other than the redundant cabs
Although, a two-car unit would normally have its traction rated to its weight, (the 309s were an exception as they were used to increase the capacity and available power of the heavily loaded service). Thus the transformer/rectifier/motor mass would be less, - but probably more than half of the kit on a 4-car unit.
 
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