Is third rail bad at acceleration?

[Recessio]

Is the problem DC conductor rail systems, or specifically third rail systems? I ask because London Underground obviously operate a four-rail DC system, and some of their units (2009, S-stock) can absolutely shift. When they run alongside NR lines you can really notice this.

 

[craigybagel]

In my experience, even sluggish DC EMUs leave their DMU equivalents standing. The main reason, apart from power to weight ratios is that the much heralded maximum power of a DMU is that of the prime mover, I.e. the diesel engine. There's no additional power available. An EMU can run its electric motors beyond their continuous rating for short periods, including whilst accelerating. They can even run them above their one hour ratings for a very short time, e.e. minutes.

It's fair to say that EPBs and HAPs took off a lot faster than the very similar class 205 DEMUs. The same applies to 377 versus 171. The EMUs were also much faster than the first gen DMUs. This was proved many times between Gatwick and Earlswood.

Thanks both. I know that whenever I've travelled on Electrostars on DC it's felt like acceleration is very slow, but I've not had any access to a speedometer to compare it to how the diesel trains I work compare.

Is the problem DC conductor rail systems, or specifically third rail systems? I ask because London Underground obviously operate a four-rail DC system, and some of their units (2009, S-stock) can absolutely shift. When they run alongside NR lines you can really notice this.

If probably helps that most LUL trains will be geared for a much lower top speed. Try racing each unit to 100mph....

 

[notadriver]

Plenty of gps speedometers to compare if you want Craigunable. The days of needing to see the train Speedo to check speed are long gone (unless you’re in a tunnel or a Voyager

 

[Samzino]

Is the problem DC conductor rail systems, or specifically third rail systems? I ask because London Underground obviously operate a four-rail DC system, and some of their units (2009, S-stock) can absolutely shift. When they run alongside NR lines you can really notice this.

LU stock are lighter but also geared to 40-62mph tops so the acceleration is much better due to the resulting gearing having higher torque than heavy rail stock that's geared for higher speeds.

 

[Nym]

LU also has a much beefier supply available and a lower loop reluctance thanks to the extra rails.

 

[hkstudent]

LU stock are lighter but also geared to 40-62mph tops so the acceleration is much better due to the resulting gearing having higher torque than heavy rail stock that's geared for higher speeds.

I think most Southeastern and Southern metro should gear to a lower speed limit like 100kph as of S stock. Given there's no section requiring running over 60mph in Greater London region anyway, which will give a better runtime performances.
I am sure 1 second saving in runtime means quite a lot even on NR land.

 

[edwin_m]

Re-gearing to increase the acceleration would make things worse for the power supply, unless it also resulted in top speeds being reduced so much that they extended journey times significantly.

If the achieved speed is the same, the amount of energy supplied to the train to reach that speed will also be the same. But with greater acceleration it would be supplied over a shorter period, making the demand on the power supply more variable and with a higher maximum.

 

[cjmillsnun]

I’d have thought 450s would accelerate quicker than 444s as 450s have 2 motor carriages and 2 trailers, whereas 444s have 2 motors and 3 trailers. But I don’t know how they compare to other stock

Agreed especially as 444s are 23m cars and 450s are 20m cars. So the 444 cars are heavier.

 

[swt_passenger]

I’d have thought 450s would accelerate quicker than 444s as 450s have 2 motor carriages and 2 trailers, whereas 444s have 2 motors and 3 trailers. But I don’t know how they compare to other stock

When someone posted the exact power capping limits a few years ago, the 444 was allowed a greater power draw than the 450, but a 10.444 still had a slightly worse performance than a 12.450. However there was supposed to have been a recent modification to ease the 10.444 power limitation to allow exactly the same performance as 12.450s. I don’t think detailed figures were made public.

 

[Sun Chariot]

... there was supposed to have been a recent modification to ease the 10.444 power limitation to allow exactly the same performance as 12.450s.

They certainly run to very similar timings on the Portsmouth Direct Line; and they're quite happy running as a 9-car (one of each type, photos: post #35) with no detrimental effect to the units or to passenger comfort.

 

[AM9]

Agreed especially as 444s are 23m cars and 450s are 20m cars. So the 444 cars are heavier.

See my post above on this matter (#13). The average weight of the 444s' 23m cars is 45.4 tonnes vs 44 tonnes for the 450s'. So the difference is negligible. What matters is the power to weight ratio for each train. A 10 car (2x5 car) 444 weighs 454 tonnes and has 3000KW of motor power which is 6.6KW/tonne. A 12 car (3x4 car) 450 weighs 528 tonnesand has 4500KW of motor power which is 8.52kW/tonne. So both are effectively too high for normal services on 3rd rail and the solution is to limit each type to give approximately equivalent power, hence the different settings in the trains' software.

 

[CarrotPie]

and they're quite happy running as a 9-car (one of each type, photos: post #35) with no detrimental effect to the units or to passenger comfort.

Surely one would at least be smart enough to allow that to happen, even if the different classes have different characteristics.

 

[irp]

In the case of 3rd rail, power supply is specced for max allowed current draw, - and when abnormal demand is made, the voltage sags ever nearer the minimum operating limit.

Basic V=IR

If current rises above what the supply can feed, then voltage will need to drop, as R [Resistance] will be a constant

 

[edwin_m]

Basic V=IR

If current rises above what the supply can feed, then voltage will need to drop, as R [Resistance] will be a constant

Also, if the substation keeps the local voltage at a constant 750V, the voltage further away will drop if trains are drawing current, and rise if they are regenerating.

 

[AM9]

Basic V=IR

If current rises above what the supply can feed, then voltage will need to drop, as R [Resistance] will be a constant

Not sure what your point is just stating Ohm's law. The fact is that a low voltage traction supply is much more vulnerable than a high voltage one when an abnormal load is applied. The protection threshold of a 3rd rail supply is very near the maximum normal current of a heavy service pattern. On a 25kV OLE, the safety threshold (i.e. inadvertent short circuit protection) is generally much higher than any operational load.

 

[SolomonSouth]

The 465/9 is one of the faster accelerating third rail units in the country. BRELS a bit slower but there isn't a huge difference until over about 45. The 465/9 can get to 50 in 39-40 seconds (could easily be 44-45 if the unit itself isn't feeling good, which is more common with BRELS but can happen with /9s still).

However the fastest 3rd rail train is without a doubt the 1995 stock. It can get 30 seconds to 50mph, which is surely much quicker than the 777 and quicker than about 99.9% of units under the overhead wires. I have a feeling it will stay unbeaten for a while.

 

[CarrotPie]

However the fastest 3rd rail train is without a doubt the 1995 stock

How can a double juice rail train be the fastest on one juice rail?

 

[SolomonSouth]

How can a double juice rail train be the fastest on one juice rail?

Another thing - it's on 630V and not 750. If it were on 750 it would easily be 5-9 seconds quicker to 50mph than it already is

 

[MCR247]

If Tube stock is included wouldn’t the 09 stock beat the 95 stock?

 

[CarrotPie]

If Tube stock is included wouldn’t the 09 stock beat the 95 stock?

Yes, but the tube uses a different voltage and a different number of juice rails to NR metals, plus they're geared for a completely different type of operation, so it's essentially comparing apples and oranges.

 

[edwin_m]

Parts of the Underground have been upgraded to 750V.

FOI request detail

FOI request detail

tfl.gov.uk

Beginning in 2016, sections of the sub-surface network were reconfigured to a 750 V configuration (−250 V and +500 V rails). As of 2019, the entire sub-surface network uses this configuration except for Uxbridge to Finchley Road (via Harrow) as 1973 Stock and 1996 Stock (of the Piccadilly and Jubilee lines respectively), share the railway.

 

[CarrotPie]

Parts of the Underground have been upgraded to 750V.

But not where the 95TS runs.

 

[Samzino]

The 465/9 is one of the faster accelerating third rail units in the country. BRELS a bit slower but there isn't a huge difference until over about 45. The 465/9 can get to 50 in 39-40 seconds (could easily be 44-45 if the unit itself isn't feeling good, which is more common with BRELS but can happen with /9s still).

However the fastest 3rd rail train is without a doubt the 1995 stock. It can get 30 seconds to 50mph, which is surely much quicker than the 777 and quicker than about 99.9% of units under the overhead wires. I have a feeling it will stay unbeaten for a while.

Tube stock are not only lighter but geared to 40-50mph. The 465 would probably time even better at such gearing due to the higher torque.

 

[hkstudent]

The 465/9 is one of the faster accelerating third rail units in the country. BRELS a bit slower but there isn't a huge difference until over about 45. The 465/9 can get to 50 in 39-40 seconds (could easily be 44-45 if the unit itself isn't feeling good, which is more common with BRELS but can happen with /9s still).

However the fastest 3rd rail train is without a doubt the 1995 stock. It can get 30 seconds to 50mph, which is surely much quicker than the 777 and quicker than about 99.9% of units under the overhead wires. I have a feeling it will stay unbeaten for a while.

But surprisingly, I have never tried Southeastern drivers who did that high-speed acceleration no matter is being on time or late or heavily later while having long green signals.

 

[Nicholas Lewis]

About a third of the return current on 3rd rail leaks away to buried assets, ground or anything metallic.

No it doesn't but it is certainly susceptible to leakage in wet conditions which are very prevalent in Sevenoaks Tunnel in places.

It has fried metal fixing in tunnels, Sevenoaks was the worst I know even melting the running rails. It dissolves OLE bases, the NLL was a pain.

The problems on the NLL were due to mixed traction as the OLE has the running rails solidly bonded back to the masts which isn't what you want to be doing with a DC railway. Various solutions were tried in an attempt to minimise leakage and the best for just to have it as AC of course.

It also turns water mains into sieves, leaking & accelerating the cycle.

It can do but the installation of Cathodic protection is employed on risky assets like gas and petroleum pipelines to mitigate against that although with plastic or plastic coated pipes the norm today its needed less.

To resolve this LUL uses 4 rail & split voltages, the ELL has copper mats to capture losses & return them to the sub station.

LUL runs +400 with -200 on negative rail to encourage the DC back where you want it and not in the tunnel lining segments.

Higher voltages arc, higher currents leak. Only OLE reduces this sufficiently but even this can make over bridge live or induce currents in parallel assets.

Higher currents don't leak its voltage that drives current and you want as high a resistance to earth as you can sensibly get on a DC conductor rail system which is why rails are insulated from sleepers as are rail fastenings. With cast iron brakes all but eliminated there is also a lot less contamination in the track area that certainly propagated leakage in the past.

 

[reddragon]

No it doesn't but it is certainly susceptible to leakage in wet conditions which are very prevalent in Sevenoaks Tunnel in places.

The problems on the NLL were due to mixed traction as the OLE has the running rails solidly bonded back to the masts which isn't what you want to be doing with a DC railway. Various solutions were tried in an attempt to minimise leakage and the best for just to have it as AC of course.


It can do but the installation of Cathodic protection is employed on risky assets like gas and petroleum pipelines to mitigate against that although with plastic or plastic coated pipes the norm today its needed less.

It's a common misconception by NR that their 3rd rail doesn't leak that much. It's not helped by the fact that all the earth returns are joined up in some areas making accurate measurement impossible. Leakage levels around New Cross were so high, that the conversion of the ELL from 4 rail to 3rd rail required a section gap long enough for no unit to bridge the gap and relies on coasting. The ELL was built with modern return trips so if the current out didn't match the current back, it trips the power. We also used copper return mats between the tracks & metal pipes.

I worked on the NLL, I don't think that any earth return existed due to the bodge job conversion from 4 rail to 3rd rail. Leakage measurements were so high monitoring equipment got fried!

I did a lot of work around Catford and a few other areas measuring stray currents. Whatever the weather, you could see passing trains on pipelines measure a mile or more away. The graphs showed train time, load & train length from a remote monitor on a pipe.

Stray currents occur at all times. The rails are often embedded in the ballast and even the 3rd rail in places. I have some pictures I took of the ballast nearly at the top of the 3rd rail.

Cathodic protection? That's a laugh, so many fried by extreme current leakage!

 

[Nicholas Lewis]

The East Grinstead Line is a classic example the electrification was specd on the the basis when the Electrostars were introduced that the Uckfield Line would always be diesel trains so there is actually no power available for the Uckfield trains to run on electric between South Croydon and Hurst Green let alone charge batteries. The effect is that Battery Uckfield trains would have to run on Battery once they left the Brighton Main Line which puts Uckfield out of range. Its not even a simple case of upgrading the power supplies because the DNOs power distribution systems in Kent and Sussex are pretty much maxed out.

East Grinstead was electrified in 1987 to deliver 8 car EP stock on a half hourly service. Its has been substantially enhanced since then so it could support 12 car 700's and given the service cut backs it could easily support an hourly Uckfield service now.

 

[ComUtoR]

But surprisingly, I have never tried Southeastern drivers who did that high-speed acceleration no matter is being on time or late or heavily later while having long green signals.

Your perception doesn't match the truth.

 

[Nicholas Lewis]

Not sure what your point is just stating Ohm's law. The fact is that a low voltage traction supply is much more vulnerable than a high voltage one when an abnormal load is applied. The protection threshold of a 3rd rail supply is very near the maximum normal current of a heavy service pattern. On a 25kV OLE, the safety threshold (i.e. inadvertent short circuit protection) is generally much higher than any operational load.

Indeed the Southern was setup originally with 7.5kA short circuit protection against a c6kA max current draw of a 12 car EP stock vs 25kV where o/l protection was around 600A with a short circuit level of 6kA. Thus plenty of scope on AC to exploit the ability of motors to be temporarily overloaded during the acceleration phase with power delivered in excess of the nominal rating of the motor without a risk of a tripping.

 

[Bikeman78]

East Grinstead was electrified in 1987 to deliver 8 car EP stock on a half hourly service. Its has been substantially enhanced since then so it could support 12 car 700's and given the service cut backs it could easily support an hourly Uckfield service now.

If Uckfield ever gets electrified then perhaps trains will split and join at Oxted. So the power requirement north of Oxted wouldn't be much greater than now.