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377s in 4, 8 and 12 formations - any differences for drivers?

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Railcar

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My journey today from East Croydon to Bognor Regis and back included 377s in 4, 8 and 12 formations.

Nothing remarkable in the way of speed on the way down, but on the way back we really flew between (roughly) Salfords and Stoats Nest on the Quarry Line.
8 became 12 at Horsham after an attachment.
Is it easier to get to and run at full line speed with 12 cars than with 4 or 8?
Or is it the same and the driver was just making up time?
 
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Nicholas Lewis

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In plain physical terms the front end of the train accounts for a increasing amount of air resistance the faster the train goes so hence the profile of high speed trains. So the advantage you gain with EMUs is that 8 and a 12 car have a surplus of tractive effort that isn't required to push the front of the train through the air so they will go faster for the same power at the wheels. Given your journey though and being a regular user of the route the Up Quarry out of Quarry Tunnel does have the added advantage of being a nice downhill run all the way to East Croydon so you always get a good turn of speed.
 

Bikeman78

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My journey today from East Croydon to Bognor Regis and back included 377s in 4, 8 and 12 formations.

Nothing remarkable in the way of speed on the way down, but on the way back we really flew between (roughly) Salfords and Stoats Nest on the Quarry Line.
8 became 12 at Horsham after an attachment.
Is it easier to get to and run at full line speed with 12 cars than with 4 or 8?
Or is it the same and the driver was just making up time?
The downhill run from the summit on the Quarry line until the brakes come on somewhere around Purley Oaks has always been a fast section. Back in the day I always used to stand next to one of the intermediate cabs on the slam door units and watch how fast the train flew down the bank. The modern units are more powerful so they can go uphill almost as fast.
 

D365

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In plain physical terms the front end of the train accounts for a increasing amount of air resistance the faster the train goes so hence the profile of high speed trains. So the advantage you gain with EMUs is that 8 and a 12 car have a surplus of tractive effort that isn't required to push the front of the train through the air so they will go faster for the same power at the wheels.
In some cases, timetable modellers will even account for this when running simulations!
 

Fincra5

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In plain physical terms the front end of the train accounts for a increasing amount of air resistance the faster the train goes so hence the profile of high speed trains. So the advantage you gain with EMUs is that 8 and a 12 car have a surplus of tractive effort that isn't required to push the front of the train through the air so they will go faster for the same power at the wheels. Given your journey though and being a regular user of the route the Up Quarry out of Quarry Tunnel does have the added advantage of being a nice downhill run all the way to East Croydon so you always get a good turn of speed.
Well.. that works for AC.

A 4 car on DC is much quicker than an 8 or 12. From driving experience.

Always prefer a 4 car to drive. Quicker off the mark and less DOO cameras to observe
 

sw1ller

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Well.. that works for AC.

A 4 car on DC is much quicker than an 8 or 12. From driving experience.

Always prefer a 4 car to drive. Quicker off the mark and less DOO cameras to observe
To add to that, the old DMUs I drive don’t tend to have any difference at all. Plus, you have the added bonus of being able to get the power down quicker at speed increases with shorter formations.
 

Railcar

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Thank you for all your thoughts. Am I correct that the line speed for the Quarry line is 90mph?

On the way to Bognor, the 8 splits to two 4s at Horsham. I sat in the front four (non-stop between Horsham and Barnham) so I could have a hot chocolate from the platform cafe at Barnham while I waited for the stopper, the part that goes to Bognor. What is the line speed for the non-stop run down the Arun valley?
 
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notadriver

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Well.. that works for AC.

A 4 car on DC is much quicker than an 8 or 12. From driving experience.

Always prefer a 4 car to drive. Quicker off the mark and less DOO cameras to observe

Indeed. Have you driven them on AC ?
 

D365

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Well.. that works for AC.

A 4 car on DC is much quicker than an 8 or 12. From driving experience.

Always prefer a 4 car to drive. Quicker off the mark and less DOO cameras to observe
That’s simply because of limitations on the DC supply. The advantage of reduced air resistance per carriage on a longer train remains.
 

Fincra5

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Indeed. Have you driven them on AC ?
No but mates who have notice the difference. I will be soon however :)

That’s simply because of limitations on the DC supply. The advantage of reduced air resistance per carriage on a longer train remains.
Very true, but whilst on DC any reduced air resistance isn't noticeable vs the lack in power due to limitations in the supply ;)
 
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D7666

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Well.. that works for AC.

A 4 car on DC is much quicker than an 8 or 12. From driving experience.

Always prefer a 4 car to drive. Quicker off the mark and less DOO cameras to observe

I think your experience may be due to current capping while on DC, and aligns with below.


When you multiple 377s, the software counts the number of motor coaches and divides the current limit between that number. I have got the values somewhere, not dug them out, so this is off the top of my head :

I /think/ it is something like a single 3car 377 is 1500 A and a single 4car 2250 A i.e. 750 A per pack.

But when you multiple them, it is not simple addition of 750s. 2 x 3car is not 2 x 1500 A but something less, and also 2 x 4car also not 2 x 2250 A, then when you add further units, it is even further less than the sum of the individual units.

It has to work like this - the nominal country current limit is 4500 A (yes ? Nick ?) if you just banged together 4 x 3car you'd get 4 x 1500 which is 6000 [more than the country limits], and 3 x 4car 6750 (which is just below the absolute 6800 A limit).

Also, there are two ways to make 12 cars - 3x4 has 9 motor coaches and 4x3 has 8 motor coaches - so the software equalises current cross the complete train to equalise timetable performance.

I may not have the Ampere figures all precise, but that is the gist of it. At least that was how it was explained to me when 375s and 377s were new.

Anyway, it is one possible answer to why a 4car 377 on DC appears to perform better than a bigger formation.

I think this principle did not apply to any of the Desiros, be they 444 or 450, when new, but in more recent years there has been a mod to for current capping but unsure what it is. There has to be something as nominally the traction pack on a 444 is the same as a 450 - and a 12car 450 has 12 but a 10car 444 8.

700s are different yet again - while these do not multiple with each other in normal circumstsances - but run solo, they too have current capping as solo units that favours 8car over 12car. GSM detects where the train is. A FLU is 6800 5000 and 4000 A for inner suburban outer suburban and country area, RLU 4500 3750 3000 A. You can see here that compared to train length, the current capping favours the shorter train.

Zero idea how all this will work with 701s.
 
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notadriver

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The SWR Desiro power constraints are so bad that the cross country Voyagers which come to Bournemouth easily outpace their DC electric brothers both during acceleration and at the higher end of the speed range.
 

AM9

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The SWR Desiro power constraints are so bad that the cross country Voyagers which come to Bournemouth easily outpace their DC electric brothers both during acceleration and at the higher end of the speed range.
All of which will eventually tip the decision to upgrade Basingstoke to Eastleigh to ac. Low voltage DC is becoming a liability in terms of performance and eventually line capacity.
 

notadriver

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All of which will eventually tip the decision to upgrade Basingstoke to Eastleigh to ac. Low voltage DC is becoming a liability in terms of performance and eventually line capacity.
I doubt that will ever happen due to the massive cost involved. If there was no existing electrification in place then perhaps.

Later classes such as 700 and 707s seem to have better performance than their older Desiro sisters.
 
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SolomonSouth

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I doubt that will ever happen due to the massive cost involved. If there was no one electrification in place then perhaps.

Later classes such as 700 and 707s seem to have better performance than their older Desiro sisters.
I timed a 700 (DC) from Higham-Gravesend on a mostly flat track - 73 secs to 60 but just 11 secs to 20. 700 (AC) took 40 secs to get to 53mph, Finsbury Park to King's Cross. How does the 700 on DC compare to a 444? And how would the 700 on AC compare to a 360/350/380?
 

Nicholas Lewis

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Well.. that works for AC.

A 4 car on DC is much quicker than an 8 or 12. From driving experience.

Always prefer a 4 car to drive. Quicker off the mark and less DOO cameras to observe
Yeah i knew i should have qualified that was the case with EP stock as this is when i did modelling for early generation traction power system modelling with Derby a few decades ago.
 

notadriver

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Yeah i knew i should have qualified that was the case with EP stock as this is when i did modelling for early generation traction power system modelling with Derby a few decades ago.

The performance difference with standard DC stock (1000 hp ) as opposed to the (4rep 3200 hp) compared with electrostars is interesting when multiple formations are considered.
 

Nicholas Lewis

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All of which will eventually tip the decision to upgrade Basingstoke to Eastleigh to ac. Low voltage DC is becoming a liability in terms of performance and eventually line capacity.
Bournemouth Electrification was low cost and the design factored in that much of the time the trains would be running at line speed which on DC motored stock means current demand is much reduced c700A/4car at 75mph. So substations were further apart and of lower power rating than Kent Coast (as an aside this is why this line is often quoted at 850V this is just a function of voltage regulation across the rectifiers needing a higher no load voltage to deliver 750v at full load). However, three phase drives and their clever software controlled VVVF inverters means they draw constant current right through the speed range which imposes a much higher power demand on the traction system and then you have to add the addition hotel load which on a hot day can add upwards of 200kw/4car or another 250A on top of traction demand. This led to two issues west of Basingstoke in insufficient substation rectifier capacity to support the load as well aggregated demand on the high voltage power system feeding the traction substations being exceeded. This is why the modern EMU's have been software current limited the longer the formation and as to whether Desiros had selective current limiting through software in multiple unit combos im unaware. Many of these constraints existed when the trains were introduced have progressively been removed by upgrades to the traction system but not across every electrified route so unless you have GPS control based on location your stuck with whatever limit has been agreed by NR when the trains were accepted.

DC low voltage can support performance but it comes at the expense of increased losses and the need for many more substations but unless you want to make this section a 125mph railway it won't be cost effective to convert to 25kV.
 
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