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Post by happybunny on Jun 11, 2008 16:18:47 GMT
OK this may seem a little far fetched .. but I was wondering why LUL don't order trains (when ordering new trains) that can run on 3rd rail only.. i.e. not needing the 4th rail!
Now this may sound silly... but the LO and SWT trains run on third rail only, on the same tracks that LUL trains run on 3rd and 4th rail.. Also the power is only 10 volts more than LUL's positive rail... so I think it would be a good idea to specify trains that just need the positive rail 420v only to run.
Eventually this would mean that the negative rail would be redundant and could be removed. Saving lots of time and money on track work etc!
It shouldn't be too tricky... maybe adjust the current 3rd/4th rails to the same specification as the Richmond/Wimbledon branch NR tracks. This way 3rd rail and 3rd/4th rail trains could work together until such a time as 3th/4th rail trains are redundant!
In the long run it would save lots of money when LUL come to doing track works and maintenance.
TBH I am surprised someone didn't think of this for the JLE.. build trains that could just run on 3rd rail only and build the extension 3rd rail powered only... would have saved £1000's
What are your thoughts ?
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mrfs42
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Post by mrfs42 on Jun 11, 2008 16:49:29 GMT
How about the need for suppression and slugging of stray currents in the signalling circuits - the cost there would be enormous? Running rails being electrically isolated from the juice rails makes for a far simpler system. 3rd rail would require loss of one (at the very least) running rail to carry the negative return current, changing all track circuits, problems with stock interoperability (3rd rail would be trapped on 3rd rail lines).
The cost saving by not having the extra rail would be lost in the extra maul of the non-system wide form of electrification. Now if there was a truly limitless chequebook there might be some merit in changing, but not until then.
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Post by tubeprune on Jun 11, 2008 18:18:50 GMT
This has been considered many times before. When I worked at Acton, one of the graduate engineers did a study for me and, as mrfs42 says, it proved too costly and too risky to make the change. It was done again more recently by Metronet, IIRC.
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Post by happybunny on Jun 11, 2008 18:39:27 GMT
Oh thanks tubeprune... at least that assured me that my idea wasn't really stupid !! ha ha
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Post by Deleted on Jun 11, 2008 19:17:37 GMT
Is this forth rail as in the Forth Rail Bridge?  [/pedantry] |
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Ben
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Post by Ben on Jun 11, 2008 20:32:16 GMT
Also the power is only 10 volts more than LUL's positive rail... so I think it would be a good idea to specify trains that just need the positive rail 420v only to run. together until such a time as 3th/4th rail trains are redundant! Thats not right, is it? Thought it was 420/-210 on 4rail and ~660/0 on 4rail shared lines? There was a thread about this somewhere. |
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Colin
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Post by Colin on Jun 11, 2008 20:39:45 GMT
Also the power is only 10 volts more than LUL's positive rail... so I think it would be a good idea to specify trains that just need the positive rail 420v only to run. You are a District line driver trained on Network Rail stuff, yeah? Since when has Network Rail had 420v DC current? Third rail lines use 750v DC ;D ;D Eventually this would mean that the negative rail would be redundant and could be removed. Saving lots of time and money on track work etc! Were you aware that there is a actually a very valid reason why LU uses 4th rail? The old cast Iron deep level tunnels have the potential to cause earth problems with the traction current supply - the negative rail is used as way to mitigate against earth problems. It shouldn't be too tricky... maybe adjust the current 3rd/4th rails to the same specification as the Richmond/Wimbledon branch NR tracks. You obviously don't know much about electrics then. You can't just change the voltage and expect everything to work as it did before - and the modifications needed to the trains (ie, traction current return through the wheel sets, full re-wire, fuse changes, etc, etc)...........the words "It shouldn't be too tricky" are not the first that spring to my mind! In the long run it would save lots of money when LUL come to doing track works and maintenance. Now that I wouldn't dispute, but I think you'd be talking seriously long term.........like at least a century. TBH I am surprised someone didn't think of this for the JLE.. I'm guessing from the replies in the thread, you can now see why it appeared that no-one thought of it for the JLE...... |
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Post by Deleted on Jun 11, 2008 22:30:01 GMT
Third rail lines use 750v DC ;D ;D Were you aware that there is a actually a very valid reason why LU uses 4th rail? The old cast Iron deep level tunnels have the potential to cause earth problems with the traction current supply - the negative rail is used as way to mitigate against earth problems. I believe the LNWR DC system is 660volts, 750volts being the Southern Railway standard. Another reason the tube stays on the four rail system is that by keeping the traction current up in the juice rails. Rather than returning it through the running rails electrolytic corrosion of the metal tunnel segments etc is reduced. |
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mrfs42
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Post by mrfs42 on Jun 12, 2008 0:08:39 GMT
Not to mention the Board of Trade requirement of not more than 7 volts potential difference between discrete conductor ends......
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Post by Deleted on Jun 12, 2008 13:47:54 GMT
The LNWR system centered on Willesden Jcn was of a nominal 630v DC as was the LER to which it was connected at a number of locations. The first section to go live was Queens Park to Willesden Jcn for the LER service and was initially fed from the LER substation at Kilburn Park. This was positive center and negative outside until the LER finished conversion to the now standard and the LNWR commissioned their substations. The voltage present in the positive rail was +420v to earth (0v) and the negative rail was -210v to earth (0v), this resulted in a potential difference of 630v between positive and negative rails. As previously mentioned the avoidance of stray currents to earth was a prime concern and could cause corrosion of the cast iron tunnel segments. The conversion during 1970 to 3rd rail was a simple case of bonding the negative rail to one running rail, which was then bonded throughout. The other running rail retained the IBJ's for the track circuiting. The positive rail was now raised to +630v to earth (0v). The Southern on the other hand adopted 750v DC to cater for the operation of express units with with higher speeds and current consumption.
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Post by max on Jun 12, 2008 17:41:47 GMT
The disadvantages of 4th rail seem to be additional complexity of pointwork, and wear and tear/potential unreliability of additional moving parts, which presumably would be exacerbated by high speed running. The advantages are the protection of cast iron tunnels (and any nearby services) from electrolysis.
This would appear to make a case for dual collection trains, 3 rail outdoors (where most pointwork and high speed running is) and 4 rail indoors (where tunnels need protection).
Given that the current ethos seems to be for total route modernisation: new trains and new signalling introduced together, it surely wouldn't be rocket science for current collection/track circuit/signalling issues to be resolved for a new build of stock. The return current either goes through the negative shoe or it is earthed = switch. Are the technicalities and logistics really any more complicated than what happens at Drayton Park, Farringdon, or wherever.
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Post by mrfs42 on Jun 12, 2008 17:50:25 GMT
It wouldn't be rocket science to cure all the compatibility issues, but with a large proportion of available money devoted to other causes (which I'm not going to enumerate here) there simply is not enough money sloshing around the public purse to pay for this.
Current rail ramps will wear, whether they are 3rd or 4th rail. If the gist of this suggestion is to save infrastructure costs on switch/crossing design, installation and maintenance by switching to 3rd rail this just is a non-starter. The cost would be astronomical for precious little return, I feel.
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Post by max on Jun 12, 2008 18:04:46 GMT
If the system is designed correctly to begin with there shouldn't be any compatibility issues. These days, inter-running of stock between certain lines is practically unheard of, so a self-contained line would be an ideal candidate. For example, Bakerloo/DC lines have been inter-running 3rd/4th rail stocks with 4th rail bonded to running rails rather than a true insulated return for years, so where are the "compatibility" issues here?
So, Bakerloo comes up for renewel, the trains run 4th rail in tunnel section, earthed north of Queen's Park. The signalling system is already set up for this arrangement. The trains can run anywhere, on 3rd rail or 4th rail systems at the flick of a switch.
Where do you get "astronomical cost" from?
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mrfs42
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Post by mrfs42 on Jun 12, 2008 18:27:58 GMT
With regard to the Bakerloo/DC signalling the signalling north of Queens Park is maintained by NR and uses a different form of relays and circuitry to what is installed at Queens Park and southbound from there.
Now, if you can cast your mind back to when the Eurostars were mooted as running into London - every single track circuit needed to be suppressed against interference from the induced AC in the traction motors. Not just every single track circuit that the Eurostars were going to run on but every single track circuit they might run on.
Any new build of stock is going to use AC traction motors, to use DC these days is simply not cost effective in terms of contact maintenance/inductive spark quenching et al. So therefore if you are going to build new stock to run on 3rd rail north of Queens Park you've got to put suppression arrangements for the whole of the rest of the Bakerloo. In the case of Queens Park alone there are something in in the order of magnitude of 32 separate track circuits - with AC traction motors all these need to be protected against stray currents that might cause 'false positives' in the signalling system. These suppression arrangements could be quite simple (on paper) by just 'double-cutting' everything - but then theres the cost of installing this double cutting: changing/verifying the bookwiring - not to mention designing the circuits. Once that's done then you can run with a 3rd rail north of QP and 4 rails south of QP.
Is it worth it?
edit: In the case of the Bakerloo there might be a cost/benefit analysis that could come down in favour of retaining DC traction motors - after all the majority of signalling is fairly new there. Cue someone popping up and telling me that the Bakerloo is already AC-induced suppressed.
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Post by Deleted on Jun 12, 2008 18:44:44 GMT
Keeping the fourth rail is the best idea in the end - but don't the new Thales systems being introduced on the Jubilee and Northern not use track circuits? If so, it would be less of a problem but I don't really see the point in the end.
The real question is: why don't we use Ganz's overhead system ;D
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Post by mrfs42 on Jun 12, 2008 19:32:44 GMT
Keeping the fourth rail is the best idea in the end - but don't the new Thales systems being introduced on the Jubilee and Northern not use track circuits? If so, it would be less of a problem but I don't really see the point in the end. Indeed, the Thales SelTrac system uses audio frequency inductive loops. Lets hope that the known DVA problems on other SelTrac systems can be ironed out, not to mention the emergency brakes!  The real question is: why don't we use Ganz's overhead system ;D Have you ever seen a Ganz wiring diagram? I've got a few and they're scarily complex! |
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Post by tubeprune on Jun 12, 2008 20:37:27 GMT
edit: In the case of the Bakerloo there might be a cost/benefit analysis that could come down in favour of retaining DC traction motors - after all the majority of signalling is fairly new there. Cue someone popping up and telling me that the Bakerloo is already AC-induced suppressed. Don't they use FS2500 JTCs or something similar on the Bakerloo? |
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Post by mrfs42 on Jun 12, 2008 20:41:07 GMT
See, I said someone would say it! I think so, but could that only be around the Baker Street area - avoiding interference with the Jubilee?
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Tom
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Post by Tom on Jun 12, 2008 20:49:34 GMT
Whole line - excluding three tracks at Baker Street and Stonebridge Park Depot.
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Post by mrfs42 on Jun 12, 2008 20:52:57 GMT
Well, in that case...... Most of my case for 'astronomical cost' has been disproven, so I'll gracefully retire. Unless anyone else can come up with any other reason against the mix of 3rd/4th rail capable stock on the Lakerboo? |
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Post by Deleted on Jun 13, 2008 5:42:10 GMT
Keeping the fourth rail is the best idea in the end - but don't the new Thales systems being introduced on the Jubilee and Northern not use track circuits? If so, it would be less of a problem but I don't really see the point in the end. I think there is more of an issue with immunity between Seltrac's frequencies and the track circuits, PAC, and 95TS AC motors, than with the DC power supply. From what I have read (I think it's on IRSE website) these problems have been solved during testing. |
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Post by mrfs42 on Jun 13, 2008 7:05:00 GMT
Keeping the fourth rail is the best idea in the end - but don't the new Thales systems being introduced on the Jubilee and Northern not use track circuits? If so, it would be less of a problem but I don't really see the point in the end. I think there is more of an issue with immunity between Seltrac's frequencies and the track circuits, PAC, and 95TS AC motors, than with the DC power supply. From what I have read (I think it's on IRSE website) these problems have been solved during testing. particularly pp16 - 18 and 24 |
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Post by Deleted on Jun 13, 2008 16:05:33 GMT
Surely by removing the 4th rail, you also remove the redundancy feature of a 4 rail system? Currently there is a 630v potential difference between the two current rails, as one is +420 and the other is -210. When one rail develops an earth fault, due to the way the system works, the other rails voltage is increased to 630v to allow trains to continue running rather than become stalled in a tunnel section. That's what I was always told anyway.
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Post by max on Jun 13, 2008 16:15:07 GMT
To assess the overall utility of the 4th rail system, answer the following question: How many other electric railways in the world use it?
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Post by mrfs42 on Jun 13, 2008 17:52:20 GMT
How many other electric railways run in small-bore cast iron tunnel?
Therein lies the root answer for the perpetuation of the 4th rail system.
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Post by Colin on Jun 13, 2008 22:30:18 GMT
To assess the overall utility of the 4th rail system, answer the following question: How many other electric railways in the world use it? That point of view makes me think of sheep!! Certain trainers are in fashion so everyone must have them. A certain song is number 1 in the charts, cos everyone must have it. So & so colours are this summers must have...... I drive a Ford - someone else might like Peugeot, or Honda or whatever. Point is, just cos no one else uses 4th rail, doesn't mean there's anything wrong with it! In fact the redundancy feature mentioned by 'Randomnumber' is a very valid point too. |
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Post by max on Jun 14, 2008 3:50:03 GMT
But if redundancy was so important, other networks would have had it AND BR would not have removed it. You think of sheep, I think of marsupials. Typical Brits, come up with a strange technology, and then get huffy that the rest of the world is too blinkered to make use of it.
How many earth leakage problems close down Southern Electric per year?
Just to check, what is the construction method in Glasgow?
Does the size of the bore actually matter, is the 3rd rail on GN&C significantly further from the metalwork than on Underground lines?
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Post by Deleted on Jun 14, 2008 6:10:23 GMT
How many other electric railways run in small-bore cast iron tunnel? Therein lies the root answer for the perpetuation of the 4th rail system. Is this not valid? |
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Post by mrfs42 on Jun 14, 2008 7:36:06 GMT
Glasgow Undergound : 11,527 yds of tunnel: 8,178 yds lined with brick and concrete, 3,349 yards with cast iron - principally under the Clyde and in a couple of other places where there is water bearing terrain.
The size of the bore does matter, the smaller the bore the greater likelihood of electrical continuity, and the concomitant risk of electrolytic corrosion.
Glasgow wasn't originally electrified, started out as an underground cable tramway, and the entire lock-and-block signalling system used no track circuits, using instead 't' irons mounted on the the wall operated by skids on the stock.
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Post by tubeprune on Jun 14, 2008 8:16:44 GMT
The original reason for the adoption of the 4th rail system not because of the cast iron tunnels. The choice was originally made for the District and the LER tube lines follwed because of joint ownership. The real reason was because the automatic signalling was originally based on DC track circuits. The isolation of the traction and signalling was regarded as essential to avoid the risk of a wrong side failure.
Another advantage is that, with a 3-rail system, a connection to earth on the positive side will cause a short ciruit and take the current off and leave all trains stalled (in tunnels probably). Having a "fully insulated" 4-rail system means that an earth on either pole will allow the voltage to remain at 630 and trains to keep running. You can track down the earth and correct it without necessarity stopping the service. It's still that way today.
Also, you can meet the old Board of Trade requirement of no more than 7 volts drop on the return side of earthed systems and allow substations to be spaced further apart. The cast iron tunnels would have played a part in this.
Don't forget the Central Line worked on the 3rd rail system until 1940. The signalling didn't use track circuits originally and when they were installed in 1912-13, they used AC. The CLR positive rail was in the centre of the 4ft.
Not forgetting the C&SLR, it too used a 3rd rail system until its conversion to LER standards in 1922.
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[/pedantry]
