Four dc / ac locations (or is it five)?

[Bald Rick]

In the days when the North London ran through to North Woolwich, services which had started in the 1980s as 3rd rail throughout later used to change four times along the way, there being two separate overhead sections which had replaced DC along the way. Can't recall the exact stations, but if you were sat under the pantograph there was a periodic considerable bang from the circuit breaker as it went up/down.

From Stratford - Richmond it was:

Hackney Wick D.C. - AC
Dalston Kingsland AC - DC
Camden Rd D.C. - AC
Acton Cemtral AC - DC

Hackney Wick to Dalston was dual electrified until 2010, but the units swapped over as they were limited to notch 2 on DC power to limit return current issues (the track circuits there were very susceptible to the D.C. return current).

Certainly back in 313 days, DC trains arriving at both Euston and Watford would occasionally change over and return empty to depot on the AC lines (at Watford using that stub on the Down Fast line). It was described in Modern Railways at the time. Does it still happen?

I remember seeing a 313 do that a long time ago at Watford, but I’d be surprised if it happened now - the fast lines are too busy. A couple of empty Services go to/from Euston on AC via the slow lines at the book ends of the day.

 

[Taunton]

Hackney Wick to Dalston was dual electrified until 2010, but the units swapped over as they were limited to notch 2 on DC power to limit return current issues (the track circuits there were very susceptible to the D.C. return current).

I had understood that when initially it was DC only, with the 2-EPB units, there was no problem, it was only when the 25kV was installed as well things started. One of the main problems was at Highbury, where after the AC was installed the stray currents started to impact on the Victoria Line down below - the NLL had been DC electric there since 1920!

 

[Bald Rick]

I had understood that when initially it was DC only, with the 2-EPB units, there was no problem, it was only when the 25kV was installed as well things started. One of the main problems was at Highbury, where after the AC was installed the stray currents started to impact on the Victoria Line down below - the NLL had been DC electric there since 1920!

Yes that’s right.

 

[73128]

I know it's not London, but is there any interchange between DC and AC lines at Reading?

No. Neither the relatively recently reinstated underpass nor the now little used connection to the main lines is electrified on either system.

 

[Beebman]

No. Neither the relatively recently reinstated underpass nor the now little used connection to the main lines is electrified on either system.

The tracks on the high-numbered platforms on the north side of the station have holes in the sleepers for 3rd rail 'pots' so hopefully that means that there's a plan sitting somewhere for DC electrification via the reinstated underpass.

 

[thelem]

'Between The North Pole (a real pub in 2012) and Wormwood Scrubs is a railway bridge.
At that point on the line something happens that only happens at three other places in London. All those other places are stations. How many can you name?'

The 'official' answer is Drayton Park, Farringdon and Acton Central.
However, I was on a Thameslink train yesterday (Sunday 16th May). On Sundays, trains do not call at City Thameslink. However, my train came to a halt in the station and the driver explained (I paraphrase) that it was to do the changeover from dc to ac. My later return train did the changeover at Farringdon and did not stop at City Thameslink.
Is it standard practice for the northbound services to changeover at City Thameslink and the southbound at Farringdon?
If it is, the 'official' answer to the question is not quite right.

The AC electrification between Farringdon and City Thameslink was added in 2009 as part of the Thameslink Programme to improve reliability. Before that changeover happened at Farringdon in both directions.

 

[Bald Rick]

The tracks on the high-numbered platforms on the north side of the station have holes in the sleepers for 3rd rail 'pots' so hopefully that means that there's a plan sitting somewhere for DC electrification via the reinstated underpass.

Either that, or they used some spare EG47ER sleepers that were hanging around.

 

[Townsend Hook]

Certainly back in 313 days, DC trains arriving at both Euston and Watford would occasionally change over and return empty to depot on the AC lines (at Watford using that stub on the Down Fast line). It was described in Modern Railways at the time. Does it still happen?

Seems to be at least one move at the Euston end which does that.

Realtime Trains - 5C73 0100 London Euston to Willesden T.M.D

Realtime Trains is an independent source of live realtime running information for the Great British railway network.

www.realtimetrains.co.uk

 

[plugwash]

The AC electrification between Farringdon and City Thameslink was added in 2009 as part of the Thameslink Programme to improve reliability.

I was under the impression it was done because the moorgate branch was cut off, so they could no longer send a train that failed the changeover to reverse at moorgate.

 

[zwk500]

I was under the impression it was done because the moorgate branch was cut off, so they could no longer send a train that failed the changeover to reverse at moorgate.

Both are correct. When Moorgate closed the only option in the event of a DC failure if the OLE had not been extended would have been to run wrong-road back through St Pancras. So the AC was extended south in 2009 to improve the reliability of the core section, because the option of using Moorgate was no longer available as part of the Thameslink programme.

 

[AlbertBeale]

Yes it is. This allows services that fail to changeover correctly to be reversed with the functioning power supply. There is a scissors crossover between City TL and Farringdon that allows trains to use either platform and reverse back to the 'right road' line. Both platforms at both stations and both lines between themare provided with both systems. Not sure if Smithfield sidings are 3rd-rail only or not.

Surely the scissors crossing is between City TL and Blackfriars? Hence trains terminating in the Smithfield sidings pick up passengers southbound on the normally-northbound platform at City TL, before crossing to the right line before Blackfriars. And a northbound train failing to make the power switchover at City TL would reverse from there, rather than having to go towards Farringdon to cross over.

I think there's a trailing crossover south of Farringdon, so a southbound train failing to make the switch there could continue towards City TL and reverse back onto the northbound Farringdon platform.

So although all tracks and platforms at and between City TL and Farringdon have both power options, for maximum flexibility, if the changeover fails at the first station arrived at (as is the practice now), in neither case does the train have to run all the way through to the following station to reverse.

 

[Bald Rick]

So although all tracks and platforms at and between City TL and Farringdon have both power options, for maximum flexibility, if the changeover fails at the first station arrived at (as is the practice now), in neither case does the train have to run all the way through to the following station to reverse.

It does if there’s another train behind, which there almost always is.

 

[Aictos]

In a word, yes.

99.9% of the Thameslink services I use always change power supply at Farringdon in both directions, the 0.1% of the Thameslink services I use change power supply northbound at City Thameslink on weekends.

As I understood it, if southbound the driver cannot change from AC to DC at Farringdon they have the option of trying at City Thameslink if that fails then they head back north, if Northbound wasn't the idea that the driver would first change from DC to AC at City Thameslink if that fails then they have the option of trying at Farringdon if that fails they head back south?

Now while that might have been the idea as I said in my first paragraph, the vast majority of all Thameslink services simply do the changeover at Farringdon regardless.

And you can tell when the driver has changed from AC to DC or DC to AC at Farringdon for two reasons, one the noise of the inverters? are different depending on what power supply is in use and two for a brief minute the power sockets in 1st Class turn off then on again.

 

[flitwickbeds]

Yes it is. This allows services that fail to changeover correctly to be reversed with the functioning power supply. There is a scissors crossover between City TL and Farringdon that allows trains to use either platform and reverse back to the 'right road' line. Both platforms at both stations and both lines between themare provided with both systems. Not sure if Smithfield sidings are 3rd-rail only or not.

I don't understand why all trains don't change at City Thameslink regardless of which direction they're travelling in? As I recall, entrance to Smithfield Sidings is only possible from the south (ie going north)?

So if a southbound train fails to change from overhead to third rail at Farringdon, they have to continue on overhead to City Thameslink then reverse into the sidings. Why not just have them changeover at City Thameslink as a matter of course?

 

[aleggatta]

I don't understand why all trains don't change at City Thameslink regardless of which direction they're travelling in? As I recall, entrance to Smithfield Sidings is only possible from the south (ie going north)?

So if a southbound train fails to change from overhead to third rail at Farringdon, they have to continue on overhead to City Thameslink then reverse into the sidings. Why not just have them changeover at City Thameslink as a matter of course?

I'd take a gander, that the more notice you have of a problem the less of an issue it might be? Say gives the chance for the signaller to ad hoc plan the return route and stop traffic on the adjacent line to get it out the way as quick as possible? Personally I never got why the whole Thameslink programme didn't move the power changeover to somewhere a little more flexible with platform arrangements, outside of the core, at least it would then effectively be clear running from one end to the other

 

[zwk500]

I don't understand why all trains don't change at City Thameslink regardless of which direction they're travelling in? As I recall, entrance to Smithfield Sidings is only possible from the south (ie going north)?

So if a southbound train fails to change from overhead to third rail at Farringdon, they have to continue on overhead to City Thameslink then reverse into the sidings. Why not just have them changeover at City Thameslink as a matter of course?

A combination of @AlbertBeale and @aleggatta. Basically, by changing southbound at Farringdon if there's a problem then the reversal at City TL can be done just that little bit quicker.

 

[OxtedL]

1. Smithfield sidings are only accessible from the northbound line
2. If you need to turn a failed changeover around in the southbound platform at City Thameslink (to return to the northbound platform at Farringdon) then you need to hold the next southbound train in Farringdon station, which means you need to know in advance.

 

[Bald Rick]

I don't understand why all trains don't change at City Thameslink regardless of which direction they're travelling in? As I recall, entrance to Smithfield Sidings is only possible from the south (ie going north)?

So if a southbound train fails to change from overhead to third rail at Farringdon, they have to continue on overhead to City Thameslink then reverse into the sidings. Why not just have them changeover at City Thameslink as a matter of course?

Failed changeover southbound trains don’t go into Smithfield, not least because Smithfield can only hold 8 car trains, and nearly half the fleet is 12 car. So if a train fails to change over, it goes to City, tries again, and if still failed runs back through the crossover on AC and ECS to Cricklewood or Hornsey.

It’s a plan that works very well.

 

[Taunton]

One wonders why the power changeover is so fraught with failure that all these precautions have to be taken.

I do fear for the reliability of Crossrail with the multiple changeovers (signalling I know, not power) along such a high-frequency route.

 

[zwk500]

One wonders why the power changeover is so fraught with failure that all these precautions have to be taken.

I do fear for the reliability of Crossrail with the multiple changeovers (signalling I know, not power) along such a high-frequency route.

It's the proposed level of service that requires these precautions being taken. If a train failed to change over and you had already sent a train up behind it you'd have to stop the job to reverse trains back onto the ECML and MML or into London Bridge and Loughborough junction to allow the failed train out of the way.

Similar precautions are in place on the West London Line - if a northbound train fails the changeover it stops before Mitre Bridge Jn (or in GTRs case stops before it changes for added safety) and can use the turnback siding or crossovers to reverse. If a southbound train fails it simply stops and reverses where it is. In both cases there's flexibility in the signalling and dual systems on both sides of the junction to avoid trapping a train. I'm fairly sure at Drayton park there's trailing crossovers, although they'll be there for the tunnels as much as the traction. Not sure what happens at Acton Central, but it's much, much less busy than any of the others.

 

[Taunton]

It's the proposed level of service that requires these precautions being taken. If a train failed to change over and you had already sent a train up behind it you'd have to stop the job to reverse trains back onto the ECML and MML or into London Bridge and Loughborough junction to allow the failed train out of the way.

A crossover just behind the changeover point would surely prevent all this.

 

[swt_passenger]

A crossover just behind the changeover point would surely prevent all this.

There are other factors involved in the planned recovery procedures. City Thameslink has bigger platforms better able to deal safely with the numbers of pax displaced off a failed 700 in the peaks.

 

[zwk500]

A crossover just behind the changeover point would surely prevent all this.

There is one. But a crossover is no use if there's no clear route to it or beyond it. So the process is arranged to ensure that in the unlikely event of a failure, signallers can hold trains in the platform for a few minutes whilst the failed train clears the line and then carry on.

 

[Beebman]

Similar precautions are in place on the West London Line - if a northbound train fails the changeover it stops before Mitre Bridge Jn (or in GTRs case stops before it changes for added safety) and can use the turnback siding or crossovers to reverse. If a southbound train fails it simply stops and reverses where it is. In both cases there's flexibility in the signalling and dual systems on both sides of the junction to avoid trapping a train. I'm fairly sure at Drayton park there's trailing crossovers, although they'll be there for the tunnels as much as the traction. Not sure what happens at Acton Central, but it's much, much less busy than any of the others.

Some years ago I was on a southbound 378 which failed to pick up DC current at Mitre Bridge and it simply reversed wrong road back towards Willesden Jct.

 

[Bald Rick]

Not sure what happens at Acton Central, but it's much, much less busy than any of the others.

Trailing crossovers in rear for both directions (albeit a little way in rear).

A crossover just behind the changeover point would surely prevent all this.

That’s what there is!

 

[big_rig]

Where does the Southern service from MK to Clapham Junction/West Croydon change over? Haven't taken the thing in well over a year or I'm sure I would've remembered - Willesden?

 

[zwk500]

Trailing crossovers in rear for both directions (albeit a little way in rear).

The nearest ones I can see are at Acton Wells Jn and South Acton in each direction, but can't see a signalled route to run wrong road?

Where does the Southern service from MK to Clapham Junction/West Croydon change over? Haven't taken the thing in well over a year or I'm sure I would've remembered - Willesden?

Immediately south of North Pole Junction. The Southern Limit of OLE is pretty much at the A40 Westway bridge, and the northern limit of 3rd rail runs to Mitre Bridge Jn (the divergence of the routes to Wembley/Willesden Jn HL).

 

[Bald Rick]

Where does the Southern service from MK to Clapham Junction/West Croydon change over? Haven't taken the thing in well over a year or I'm sure I would've remembered - Willesden?

As mentioned above, Between Mitre Bridge Junction and Westway.

The nearest ones I can see are at Acton Wells Jn and South Acton in each direction, but can't see a signalled route to run wrong road?

Not a signalled move from Acton C platforms so done by special arrangements, but there is a shunt move from the down to the up at Acton Wells once you get to it (from AW372)

 

[Andrew S]

as I said in my first paragraph, the vast majority of all Thameslink services simply do the changeover at Farringdon regardless.

In my experience travelling northbound the changeover always happens at City Thameslink.

 

[baz962]

The nearest ones I can see are at Acton Wells Jn and South Acton in each direction, but can't see a signalled route to run wrong road?


Immediately south of North Pole Junction. The Southern Limit of OLE is pretty much at the A40 Westway bridge, and the northern limit of 3rd rail runs to Mitre Bridge Jn (the divergence of the routes to Wembley/Willesden Jn HL).

If you mean at South Acton to come back it's signalled both ways on the down platform.