Third rail or 'coasting' from an electrified junction to the next station?

[JonathanH]

GWR are meant to introduce 769s on the North Downs Line at some point in the future. The distance from Aldershot South Junction to North Camp is 1 mile 17 chains. The distance from Shalford Junction to Shalford is 58 chains.

Given these units can't change power source on the move, from a technical point of view how feasible is it to 'coast' from the junction at line speed to North Camp or Shalford and start the diesel engines at those stations.

What would be the reason not to do it if it is technically feasible? Is there some loss of breaking capability

There have been some planned instances of trains coasting through possessions in the past - eg at Biggleswade for an ECML possession some time ago - see https://www.railforums.co.uk/thread...insbury-park-1st-jan-2013.77113/#post-1315414. There are also neutral sections on electrified lines. Is this different?

Secondly, ignoring the ORR issues on extending third rail, could third rail be put down on these stretches of route by just 'laying it down' and connecting it to the existing installation at relatively low cost or would it still require a lot of development work and planning?

In the same manner, could an electric locomotive with the signals set for a clear route, coast from Acton Wells Junction to Acton East Junction?

 

[CarrotPie]

GWR are meant to introduce 769s on the North Downs Line at some point in the future. The distance from Aldershot South Junction to North Camp is 1 mile 17 chains. The distance from Shalford Junction to Shalford is 58 chains.

Given these units can't change power source on the move, from a technical point of view how feasible is it to 'coast' from the junction at line speed to North Camp or Shalford and start the diesel engines at those stations.

What would be the reason not to do it if it is technically feasible? Is there some loss of breaking capability

There have been some planned instances of trains coasting through possessions in the past - eg at Biggleswade for an ECML possession some time ago - see https://www.railforums.co.uk/thread...insbury-park-1st-jan-2013.77113/#post-1315414. There are also neutral sections on electrified lines. Is this different?

Secondly, ignoring the ORR issues on extending third rail, could third rail be put down on these stretches of route by just 'laying it down' and connecting it to the existing installation at relatively low cost or would it still require a lot of development work and planning?

In the same manner, could an electric locomotive with the signals set for a clear route, coast from Acton Wells Junction to Acton East Junction?

Three issues here. Number one, if a train gets gapped, that ain't good (I know they have diesel engines but they might not work when you need them two.) Number two, no power means no PIS, lights etc without battery/aux. Three, there would be loss of braking capacity (lack of rheo/regen brakes), but the 769s are I believe still disc braked only. So in short, yes, but lots of red tape in the way, and it would only really work for 769s. Also, freight wise, you probably wouldn't let a 1,000-tonne freight train coast with only air brakes, would you?

 

[GB]

Also, freight wise, you probably wouldn't let a 1,000-tonne freight train coast with only air brakes, would you?

Why? Freights only have air brakes.

 

[Bletchleyite]

Also, freight wise, you probably wouldn't let a 1,000-tonne freight train coast with only air brakes, would you?

What third rail hauled freight is there?

 

[CarrotPie]

What third rail hauled freight is there?

None, really.

 

[AM9]

... but the 769s are I believe still disc braked only. ...

The 769s almost certainly retain the Rheostatic braking from their life as 319s.*
They have full mechanical brakes (as would any train fitted with regen/rheo loaded braking as they need then in case motor regen isn't working and for low speed braking. So the rheo braking should be available when the DC bus is fed from the diesel gensets.
The 319s were always fitted with rheo braking from new. The mechanical brakes would have been hammered on TL services otherwise.

 

[CarrotPie]

In that case, coasting would disable thosr

 

[Bletchleyite]

In that case, coasting would disable thosr

Then they can use the friction brakes. With regard to what braking freight has, that would just be the friction brakes anyway. Any diesel train (other than some DEMUs like Voyagers) just has friction braking, nothing else.

I'm not totally sure it's a good idea (just stopping to change over like e.g. the WCML Southerns do is easier and probably carries less operational risk), but I'm not sure braking is the reason why I think that.

 

[AM9]

In that case, coasting would disable thosr

Coasting (or the lack of a supply for the DC bus) wouldn't prevent the use of rheostatic braking. The resistors are still there to load the motors.

 

[randyrippley]

Considering electric trains often coast to the next station in the event of a failure (or at least the loco hauled ones did) I don't see the problem.

 

[Domh245]

The 769s almost certainly retain the Rheostatic braking from their life as 319s.*
They have full mechanical brakes (as would any train fitted with regen/rheo loaded braking as they need then in case motor regen isn't working and for low speed braking. So the rheo braking should be available when the DC bus is fed from the diesel gensets.
The 319s were always fitted with rheo braking from new. The mechanical brakes would have been hammered on TL services otherwise.

I've never heard of 319s having rheostatic braking before? There certainly isn't any sort of motor noise as you'd expect to hear under rheo braking, and I can't find any mention of them having it elsewhere

 

[CarrotPie]

I've never heard of 319s having rheostatic braking before? There certainly isn't any sort of motor noise as you'd expect to hear under rheo braking, and I can't find any mention of them having it elsewhere

I agree! The Little Book of Numbers states only disc braking for 319s and 769s.

 

[Bald Rick]

It’s entirely possible on a technical basis - it’s just the same position as if the power trips off. Braking / doors etc would not be affected.

Whether it is desirable on an operational basis is a different matter.

I suspect it would be possible to have a short extensions of 3rd rail to facilitate, and it could be relatively cheap. As long as it is short, and not too far from a substation, you wouldn’t need another one (but you might need some electrical control equipment at the ‘far’ end). It would still need development and planning, as there are interfaces with signalling and track that would have to be designed.

 

[MarkyT]

GWR are meant to introduce 769s on the North Downs Line at some point in the future. The distance from Aldershot South Junction to North Camp is 1 mile 17 chains. The distance from Shalford Junction to Shalford is 58 chains.

Given these units can't change power source on the move, from a technical point of view how feasible is it to 'coast' from the junction at line speed to North Camp or Shalford and start the diesel engines at those stations.

What would be the reason not to do it if it is technically feasible? Is there some loss of breaking capability

There have been some planned instances of trains coasting through possessions in the past - eg at Biggleswade for an ECML possession some time ago - see https://www.railforums.co.uk/thread...insbury-park-1st-jan-2013.77113/#post-1315414. There are also neutral sections on electrified lines. Is this different?

Secondly, ignoring the ORR issues on extending third rail, could third rail be put down on these stretches of route by just 'laying it down' and connecting it to the existing installation at relatively low cost or would it still require a lot of development work and planning?

In the same manner, could an electric locomotive with the signals set for a clear route, coast from Acton Wells Junction to Acton East Junction?

In my view, an excellent case for a minor extension of 3rd rail conductor as far as North Camp platforms. Seems odd that in the recent rebuild the 769s were not enabled for on-the-move power switching. Are you sure about this? I realise when they were 319s they only ever switched between AC and DC when stationary at Farringdon. Without a DC extension, some batteries added to the 769s could also allow the units to remain in electric mode as far as North Camp before starting engines there and the converse in the other direction.

 

[JonathanH]

Without a DC extension, some batteries added to the 769s could also allow the units to remain in electric mode as far as North Camp before starting engines there and the converse in the other direction.

That is just weight though isn't it - batteries and a diesel generating unit would make them even slower.

 

[big all]

shouldn't be a problem coasting unless the electronics interfere
used to have vans from Redhill to Tonbridge and indeed 9 car thumpers as extras when Tonbridge had a blockade for Channel Tunnel work.
knowing we had to wait for a platform at Tonbridge used to power to exiting Godstone tunnel and shut the engine/s down doing around 60 and coast dead to the home signal and still be doing around 25mph
As the priming pump was running the perhaps full 15-18mins to the signal the engines would start instantly at the signal or on the move as required ??

 

[MarkyT]

That is just weight though isn't it - batteries and a diesel generating unit would make them even slower.

You wouldn't need very much battery capacity and thus weight to only cover for the two km or so involved, and the batteries could also help starting away from other non-electrified stations. Adding batteries (within limits) could plausibly IMPROVE performance, as GWR/Hitachi are projecting with their west country 80x trial. It is a shame the units don't do rheostatic braking as if provided the battery could also harvest braking energy for subsequent reuse. Something else that rationally ought to have been added to them as part of the rebuild, or perhaps another sign they weren't the best units to convert to bi-mode in the first place!

 

[zwk500]

Given these units can't change power source on the move, from a technical point of view how feasible is it to 'coast' from the junction at line speed to North Camp or Shalford and start the diesel engines at those stations.

Is there a technical reason they can't change power on the move, or is it because GWR perfer it operationally?

There have been some planned instances of trains coasting through possessions in the past - eg at Biggleswade for an ECML possession some time ago - see https://www.railforums.co.uk/thread...insbury-park-1st-jan-2013.77113/#post-1315414. There are also neutral sections on electrified lines. Is this different?

Coasting through possessions was a case of coast vs no train, which is slightly different. Neutral Sections on OLE lines are typically only a few metres.

In the same manner, could an electric locomotive with the signals set for a clear route, coast from Acton Wells Junction to Acton East Junction?

Yes, in theory. But only in that direction (i.e. Downhill), which makes it operationally rather useless.

 

[Snow1964]

I vaguely remember for a while there was conductor rail on the Salisbury line from Worting Junction towards Battledown flyover.
Similarly from Northampton Junction towards Southampton old docks.

It did mean you could run an electro diesel loco a bit further on electric power.

Also suggests the switchgear at junctions was regularly installed to power up any extensions to the electrification as an existing substation might be used for first mile or so.

My memory might be playing tricks, but I am sure I have read in a book that places like Dorking when electrified, had provision for adding the Deepdene tracks. But can’t remember which book now.

 

[Gloster]

I vaguely remember for a while there was conductor rail on the Salisbury line from Worting Junction towards Battledown flyover.
Similarly from Northampton Junction towards Southampton old docks.

It did mean you could run an electro diesel loco a bit further on electric power.

Also suggests the switchgear at junctions was regularly installed to power up any extensions to the electrification as an existing substation might be used for first mile or so.

I think that the the conductor rail on the Salisbury line at Worting Junction was in case an electric overshot the protecting signal. (It shouldn’t happen, but it does. If there were minimal cost implications in adding an extra bit of conductor rail, then prudent planning says ‘let’s do it’.) Northam may have been for the same reason, or for loco changing on trains heading for the docks.

 

[AM9]

I agree! The Little Book of Numbers states only disc braking for 319s and 769s.

Ooops, sorry guys, getting something mixed up in my memory. It's been 5 years since I went on a 319.

 

[Greybeard33]

Ooops, sorry guys, getting something mixed up in my memory. It's been 5 years since I went on a 319.

Lucky you!

 

[AM9]

Lucky you!

I'm fairly neutral about them. They gave good service for many years, but the demand of their TL duties needed the special capabilities of the class 700s.
I also did a couple of journeys in the NW on them and they were a great improvement on the previous stock there.

 

[Greybeard33]

I'm fairly neutral about them. They gave good service for many years, but the demand of their TL duties needed the special capabilities of the class 700s.
I also did a couple of journeys in the NW on them and they were a great improvement on the previous stock there.

Well, to try and bring the thread back on topic, I will just say that it is a pity that the 769 reincarnation of the 319 cannot switch from electric to diesel power on the move. Then the question about coasting would not arise.

 

[zwk500]

Well, to try and bring the thread back on topic, I will just say that it is a pity that the 769 reincarnation of the 319 cannot switch from electric to diesel power on the move. Then the question about coasting would not arise.

Is there a physical/mechanical restriction on switching on the move or is it an operational decision by GWR?

 

[AM9]

Is there a physical/mechanical restriction on switching on the move or is it an operational decision by GWR?

Even it the trains themselves can change on the move, the GWR might have reason to not want moving transition to ac at the western end, - I would expect that would be because the infrastructure might not be suitable. Maybe the diesel to DC transition hasn't been seriously looked into.

 

[Greybeard33]

Is there a physical/mechanical restriction on switching on the move or is it an operational decision by GWR?

I don't know the reason for it, but the restriction on changeover on the move was first reported more than two years ago:

Depending how the control circuitry is wired, Diesel to DC should be possible as they’re effectively the same as far as the train is concerned.

It will be but a second bus line is being installed to do it, the original DC bus being used for the Diesel side. Changeover between modes is only done when stationary.

 

[AM9]

I don't know the reason for it, but the restriction on changeover on the move was first reported more than two years ago:

Isn't the second bus line for ground when on diesel as they can't rely on unelectrified track to provide the continuity?

 

[Greybeard33]

Isn't the second bus line for ground when on diesel as they can't rely on unelectrified track to provide the continuity?

@Clarence Yard 's post above described a second 750V bus line to link up the third rail shoes, independent of the original 319 750V bus that has been connected to the diesel gensets instead of the shoes on the 769. This second bus line is only fitted to the GWR units, as the Northern and TfW examples have the shoes removed. I imagine there is a contactor to connect the two bus lines together in third rail mode, when the gensets and transformer-rectifier will be disconnected from the main bus.

I expect there is also a low resistance DC ground return connection between each pair of vehicles.