Cost of Third Rail Electrification

[Trog]

Some sleepers are longer than others, types older than the F40 tending to be longer, but sometimes with sloped ends. The newer Fastclip sleepers are longer than the F40's which were made shorter so they could travel to site on Salmons under less stringent loading conditions than the traditional 8'-6" sleepers.

There can also be a problem with drilling into sleepers, in that you have to be sure the hole you are drilling will miss the reinforcing.

 

[apk55]

Thanks for the info HSTEd & MarkyT

The steel used for running rails is obviously chosen for wear resistance rather conductivity and a large low resistance reinforcing return conductor would be sensible.

Voltage drop also represents inefficiency and an 80V drop means that 10% of the power is going into heating the rails.

Then there is the problem of fault detection. For example on a worst case section if a short occured outside a substation then that substation would trip, but since normally sections are fed from both ends the substation at the far end may see little more current than a train starting outside.

 

[swt_passenger]

Interesting detail associated with this discussion is that the North Downs line section south of Wokingham at least as far as Blackwater, (there may be more I haven't been further for a good few years) has concrete sleepers with the built in third rail pot attachment fittings already.

Looks like someone has been thinking of the future on that section.

 

[HSTEd]

You could always bias the running rails to something like -40V, that way you would get 100V of 'rise' before you hit the dangerous values, rather than just 60.

 

[AM9]

You could always bias the running rails to something like -40V, that way you would get 100V of 'rise' before you hit the dangerous values, rather than just 60.

That would mean a massive voltage tap at the feeder points.

 

[HSTEd]

That would mean a massive voltage tap at the feeder points.

The feeder point would have a 40V greater voltage than otherwise - it would actually increase current distribution efficiency, after all the only thing the train knows is that the hot and cold rails of the traction supply are a certain voltage apart - it doesn't know (within reason) what th voltage of either rail is related to ground.

750V nominal becomes 790V nominal.

 

[edwin_m]

How would you electrically separate a section where the running rails are at 40V below ground from one where they are nominally at ground, while still allowing trains to go from one to the other? I can't see any way of doing this without having switchgear that operates whenever a train passes.

 

[HSTEd]

Wouldn't the same mechanism that seperates track circuits work? It's only 40v after all. The supply can probably tolerate the current inflow through the trains grounding bus for the few sefonds it takes. It tolerates similar partial shorts when trains cross sections today.
EDIT:

Alternatively you could have a length of track longer than the train that was nominally maintained at zero volts but was only connected to the zero volt section through a diode, positioned so that current can through from the intermediate section into the normal one but not the reverse.
When the train bridges the intermediate and 0V section the system functions as normal (SiC Schottky diodes will keep the Voltage drop to ~1.2V) but when the train bridges the intermediate and -40V section the intermediate section will drop to -40V but it will not be electrically connected to the zero volt section.

No switchgear, only a box full of passive semiconductor components.

 

[Olaf]

I have been trying to find some actual supporting documentation for the estimate given by Network Rail for electrifying the Borderlands line at DC (£207m) however the number just seems to appear in the Merseyside RUS saying analysis has shown it.
But there appears to be no actual documentation of how the estimate was derived.

It also seems to be drastically out of step with estimates from Delta Rail developed as part of their low cost branch electrification work from a couple of years back.

So does anyone have information?

I have put in a request to their transparency system but since they are not bound to respond I doubt they will actually reply as it appears these estimates have been deliberately blown up.

I've also looked for the figures, but did not find them at the time. The estimated cost was not only for the electrification but for other items of work that was thought essential if the line was to be electrified.
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So I was correct in my assumption - Network Rail deliberately installed steel sleepers to ensure that third rail electrification could not take place?
How long ago was this?
The project will get cheaper and cheaper as time goes by as the date for scheduled sleeper replacement closes in.

That is stretching the rules of credibility; as indicated, there was no reason for NR to have assumed that the line would be electrified - it fails on the patronage figures and and the basic electrification criteria before you even look at the possible costs.