why is there a limit on how many units can work in multiple?

hobbm013

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Why is there a limit on how many units can work in multiple? Is it something to do with some systems operating between them or something else?

Would be grateful to anyone who knows
 
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_toommm_

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Sorry if this is off-topic, someone in the know can message me if that’s more acceptable, but why is there a limit on how many units can work in multiple? Is it something to do with some systems operating between them or something else? Would be grateful to anyone know who’s
I believe it's due to signal degradation (although that might just be conjecture) as it travels through the couplers through the various units. Of course, if you lose electrical connectivity, you lose the ability to control any train(s) beyond that point.
 

hobbm013

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I believe it's due to signal degradation (although that might just be conjecture) as it travels through the couplers through the various units. Of course, if you lose electrical connectivity, you lose the ability to control any train(s) beyond that point.
I thought it would be something to do with that, thank you!
 

Skie

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What units in particular? There can be any number of reasons, for modern units it can just be a case that the software was never designed to handle formations beyond a certain number.

There have been some spectacularly long trains formed from multiple multiple units when failures are involved. Merseyrail once had a series of unfortunate incidents that led to something like a 24 car 507/508 beastie. There is video of an 18 car move that was simply used to reposition a lot of units in one go after disruption:
 

Doomotron

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In a video I often talk about, a set of 3 Class 375s was rescued by another set of 3 375s, making an 11 coach train. Only one of the units was actually powered because - I think - it would otherwise trip some fuses or something.
 

alangla

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I’m sure I read elsewhere that with BSI coupled units, so basically anything diesel between a Pacer and a Turbostar, there’s a max number of cabs that can be in the formation, I think it’s 8. As a result you can tie 4x153s together, but potentially 12x170s
 

aleggatta

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Multiple factors determine what and how units can work in multiple, whether multiple working can occur in passenger service, if speed restrictions are imposed,. Example one I’ll use an electrostar. Maximum length in passenger service is 12 car due to software limitations. Longer formations are possible for rescue purposes in a fully coupled state. The rule regarding turbostars/sprinters varies toc to toc, I think southern limit it to 3 units in multiple in passenger service, I think I’ve heard other operators using an ‘8 cab’ rule, so there can’t be more than 8 cabs in one consist.

realistically the most limiting factor for a DMU is the reduced auxiliary voltage, with it being 24v only high resistance contacts can cause real issues, whereas on EMUs standard auxiliary voltage is 110v.

current limit index is a very valid point too. I remember a Big Bang on shed of the dc contractor when a 12 car 313 was attempted to be moved in one lump, modern emu’s are likely toget more forgiveness from the supply in longer formations as the inverter drives give a soft starting current, and slow acceleration is normally observed after a failure.
 

PHILIPE

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I’m sure I read elsewhere that with BSI coupled units, so basically anything diesel between a Pacer and a Turbostar, there’s a max number of cabs that can be in the formation, I think it’s 8. As a result you can tie 4x153s together, but potentially 12x170s

You can tie 5 x 153s together a case of which happened on Tuesday and Pacers max 4 in the formation (8 cabs)
 

Tomnick

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I’m sure I read elsewhere that with BSI coupled units, so basically anything diesel between a Pacer and a Turbostar, there’s a max number of cabs that can be in the formation, I think it’s 8. As a result you can tie 4x153s together, but potentially 12x170s
Twelve cabs maximum in that case - but intermediate vehicles have to be counted as a cab for this purpose too. It’s down to voltage drop on the control circuits, as I understand it.
 

apk55

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Traditional multiple units used a bundle of cables running the length of the train crossing between units either via multi pole plug and socket cables (old units) or auto couplers on more modern units, The drivers controls energized these wires in a sequence and each power car control equipment or power door control would tap into this bus with typically a relay (or ep valve for door controls) to actuate the equipment. If the train got too long with too many equipment's then either the power supply to the drivers desk could be overloaded or the voltage drop at the end unit could become too great and fail to operate.
 

37057

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Class 185 have a maximum of twelve vehicles. The communication systems (PIS for example) have a limiting factor but given you only see them as single or double sets in service it's never been a problem. I formed a nine car set once for testing modifications to the PIS and even moved it around the depot, purely for the novelty - vary rare to have had the chance otherwise!

Ref previous post regarding DMU control voltages, there are some types out there that use 110v circuitry too, 185s being one of them (though the driveline circuits are 24v) I think it depends on the manufacturer as I believe 195s are 24v.
 
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306024

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Back in the day were there some peak workings through London Bridge that were formed of 6 x 2 HAP? Or has memory failed?
 

36270k

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On the Southern Region , the limit was the 3rd rail current .
With 1950/1960 units 12-car was the limit e.g 3 x 4VEP or 6 x 2EPB
Only 2 Class 73's could MU in electric mode , 3 x 73 would blow the substation breakers.
 

gimmea50anyday

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TPE have ran 9 car services before, albeit rare. Manchester when a Glasgow based football team was playing in Manchester (can’t remember the fixture) and the screen went down in Piccadilly gardens, masses of Scots decamped to any pub they could find showing the game but as many were staying overnight in Blackpool TPE ran a 9 car and 6 duplicate with the 9 not calling at occy road. The 6 car still left people behind!
I have also ran a 9 car via Calder valley although in that case rear 3 were locked out. Some sort of disruption positioning move can’t remember the details but saved a cancellation
 

driver9000

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Twelve cabs maximum in that case - but intermediate vehicles have to be counted as a cab for this purpose too. It’s down to voltage drop on the control circuits, as I understand it.
Which is why it's better to think of vehicles rather than cabs because the centre car of 144s and 158s has caused confusion in the past over the number of units making up a train. A 153 counts as 2 vehicles for the sake of multiple working.
 

37057

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TPE have ran 9 car services before, albeit rare. Manchester when a Glasgow based football team was playing in Manchester (can’t remember the fixture) and the screen went down in Piccadilly gardens, masses of Scots decamped to any pub they could find showing the game but as many were staying overnight in Blackpool TPE ran a 9 car and 6 duplicate with the 9 not calling at occy road. The 6 car still left people behind!
I have also ran a 9 car via Calder valley although in that case rear 3 were locked out. Some sort of disruption positioning move can’t remember the details but saved a cancellation
That's interesting - cheers. I hadn't realised it had been done in the past out in the wild. Certainly rare to do it on depot as they don't really fit anywhere and splitting and positioning them afterwards was a bit of a pain to do!
 

Anvil1984

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Which is why it's better to think of vehicles rather than cabs because the centre car of 144s and 158s has caused confusion in the past over the number of units making up a train. A 153 counts as 2 vehicles for the sake of multiple working.
Indeed at our place it was 12 vehicles if it was all 15x, if it was all 14x or a mixture or 14x and 15x it was 8 max. However a 153 counted as 2 vehicles. All to do with the signal through the electrical boxes on the coupler I believe
 

Jim Jehosofat

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On the Southern Region , the limit was the 3rd rail current .
With 1950/1960 units 12-car was the limit e.g 3 x 4VEP or 6 x 2EPB
Only 2 Class 73's could MU in electric mode , 3 x 73 would blow the substation breakers.
Not quite right, we used to regulary operate 13 car, sometimes 14, on boat trains from Victoria to Folkestone Harbour or Dover Marine, usually 3 x 4 CEP + 1 or 2 MLV. Sometimes it would be a CEP/BEP/CEP formation + the MLV
 

big all

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Not quite right, we used to regulary operate 13 car, sometimes 14, on boat trains from Victoria to Folkestone Harbour or Dover Marine, usually 3 x 4 CEP + 1 or 2 MLV. Sometimes it would be a CEP/BEP/CEP formation + the MLV
the current index loading limit was 16 any more you cut out motors
a class 73=8units
a rep =14 or may be the full 16 cant remember
also you could have 2xmlv[motor luggage vans ] rather than an mlv and tlv[trailer luggage van] and a 12 on a boat train and not exceed the 16 limit
all other stock up until say pre 1990 are all 2 or 4 dependent on the number off traction motors but in general 1 per coach except as said above the reps and mlvs.
although i did train on the319s i cant remember what current limit they where as they had a bit more go in them.
 

gimmea50anyday

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As I understood it 4CEP+1MLV was within the typical third rail index of 14.and I do state typical as not every route is powered equally. An EE507 is worth 1 but an EE546 was 1.5 so a 12CEP plus 1 MLV would be rated 14, REP+CEP would have an index of 16 and required motors to be isolated and I recall stories on this forum this has been done. 3x73=18 so definitely popping breakers unless running on diseasal. Sure it wasn’t a TLV coupled to the MLV?

It is possible however the boat train route had a higher index, especially once additional power was added for Eurostar, but by then the point in boat trains was lost...
 

CW2

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Back in December 1988 all class 155 units were withdrawn from service due to issues with the doors opening at speed. (They were subsequently rebuilt into the class 153 fleet). To overcome the sudden shortage of units, a number of class 156 DMUs were hooked together and ran ECS from Inverness to Cardiff Canton. Unfortunately I have no record of how many units were moved - but I do remember seeing the train arrive at Cardiff on 16/12/88 and I think it was either 5 or 6 units (10 - 12 cars).
 

driver9000

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Indeed at our place it was 12 vehicles if it was all 15x, if it was all 14x or a mixture or 14x and 15x it was 8 max. However a 153 counted as 2 vehicles. All to do with the signal through the electrical boxes on the coupler I believe
That's right and the voltage came from batteries on the rear vehicle.
 

Eccles1983

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I was told it was brake units you count.

So sprinters was max of 12. Unless you had a 153 in it then it was limited to 10.

If a 142 was involved then it's a max of 8.
 

Llama

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As far as I'm aware with Sprinters/Pacers it's not so much to do with the signals through the coupler but the number of brake control units in the train, and the resistance that each additional brake control unit and associated circuitry has on the supply from single start battery in the consist that is used to energise the relevant train wires for brake control. As was said upthread, in Sprinters/Pacers in normal circumstances that's usually the rearmost start battery in the train. Obviously those brake control circuits are the most safety critical circuits in the train, which is why they have an absolute bearing on the number of vehicles permissible.

So what happens if more than the maximum are coupled on 14x/15x? Some systems start to play up, for instance door control won't work, rather like in older EMUs when battery charging is lost and the battery voltage begins to drop just before 'low battery volts' protection activates.

The 'number of cabs' isn't quite right and maybe ignores or maybe originated from a time before the three car 144/158 units were around. Or perhaps it was a phrase that stuck when the 153s appeared.

More modern units are limited by the software processing abilities of their train management computers and the way the various systems such as the TCMS, Asdo, CCTV, passenger alarm systems, fire detection systems etc are able to function and present information to the driver - e.g. on a 195 the maximum number of units possible in multiple in normal working is four units (irrespective of whether two or three car units). A fifth unit can be coupled but must have the TCMS isolated, which isolates a lot of other systems on the train, so passengers wouldn't be allowed to travel on it.
 
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D7666

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As I understood it 4CEP+1MLV was within the typical third rail index of 14.and I do state typical as not every route is powered equally. An EE507 is worth 1 but an EE546 was 1.5 so a 12CEP plus 1 MLV would be rated 14, REP+CEP would have an index of 16 and required motors to be isolated and I recall stories on this forum this has been done. 3x73=18 so definitely popping breakers unless running on diseasal. Sure it wasn’t a TLV coupled to the MLV?

It is possible however the boat train route had a higher index, especially once additional power was added for Eurostar, but by then the point in boat trains was lost...
Rep+Cep was certainly done. Not only was I on board it, I chartered it. It wasn't planned, it was a SEG solo Rep tour, but the unit suffered an m.g. failure such that we had no train lights, so a Cep was attached to, in theory, allow the move of passengers into the lighted Cep, but is this all this was down in deepest Kent with special crew, they still had to get the Rep back to the SW, so it remained together. Both units were defintiely under power. Allegedly we did trip one breaker somewhere. Rep+Cep = index 18.

It should also be recorded that in the small hours of the night 16EPB and up to 20EPB ECS moves took place on SE side between Slade Green and Grove Park with index 16 and 20 respectively. Not booked, but done under arrangement with control and pcr.

Of course both examples are exceptions toprove the rule, the rule was 16.


WRT 373s, the conductor rail index system does not apply. Indices apply only to DC motor units, so, with todays railways, only 455 456 313, and, for now, unretractioned 442.


Additional power was added for Eurostar it increased nominal current limit to 6800 Amp BUT did not allow an increase in conductor rail index for DC motor units. A similar power upgrade occurred when 465s came in on the suburban SE side, but again this did not allow permit DC motor trains an overall index increase.
 
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D7666

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the current index loading limit was 16 any more you cut out motors
a class 73=8units
a rep =14 or may be the full 16 cant remember
also you could have 2xmlv[motor luggage vans ] rather than an mlv and tlv[trailer luggage van] and a 12 on a boat train and not exceed the 16 limit
all other stock up until say pre 1990 are all 2 or 4 dependent on the number off traction motors but in general 1 per coach except as said above the reps and mlvs.
although i did train on the319s i cant remember what current limit they where as they had a bit more go in them.
Supporting and agreeing all you say :

319s were index 4.

They were the last "all new" emu type to get an index ... in strict chronological order 456s were newer. but in this respect a 456 isn't an "all new" type but half a 455 [albeit with 319 type traction control].

You are also correct with boat trains with 2 x MLV.

TLV post dated MLV by several years. Boat trains in 1960s did to operate 12Cep+2MLV at the index limit of 16. As more and more boat trains were needed as traffic grew, and increasing traction trips, the TLV came into being.


Rep were indeed 14.

It was permitted to couple and multiple a 2car such as a 2Hap. I do not know of Rep+Hap occuring BUT Rep+Ice did. In 1970s the last day time Up 93 from Bournemouth that used to only run to Basingstoke traditonally was not a Vep but a solo 4Rep (this then berthed at Basingtsoke to attach to the next day 8TC 06:blip Salisbury Waterloo); on occasion that train from Bomo was formed 4REP + 2car de-icer; I have two such haulages with two from the original Bomo line de-icer batch 001-003.
 

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