If 8 are usable part of the way can they not use all 8 then the driver
walk along and lock the back 2 out?
It'll be the guard locking the doors out. All operations relating to the doors on 171s on the Uckfield Line (in normal service) are down to the guard.
With 8-car formations including a 2-car unit on the rear going Southbound, the rear 2 coaches will not have access to platforms after Hurst Green (except of course Uckfield when it is open) and therefore passengers in these coaches will be asked to leave and they will be locked out at Hurst Green. The reverse applies if there is a 2-car unit at the front on arrival at Crowborough (or Uckfield, etc. etc.) - that would be locked out and then reopened at Hurst Green.
Or at least, that's how the theory goes.
Dwell times at Hurst Green are often problematic on longer formations anyway; trains invariably arrive at Hurst Green at least a minute or two late, and then there are dispatch sighting issues for longer trains. Add the process of unlocking or locking a 2-car unit at the rear of the train, and you rapidly start running out of time. Trains are not always regularly spaced at Hurst Green and run on a section of line with three-aspect signalling and 6tph frequency in the peaks (this is the busiest section of the Oxted routes) and so any delays from faffing around with a t-key in the internal door control switches will inevitably lead to everyone else being late there, and at Ashurst, and at Crowborough. So, really, trains may as well have no more than 6 coaches in use to start with, if there's a 171/7 2-car on the back.
It's all ultimately down to Control though. They may decide they want the rear 2 split from the train, or to leave them in service to Oxted or Hurst Green. Or they may just advise, as above, to leave them empty. If they don't say anything, then the guard will decide when they get taken out of use en-route.
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The need for the longer platforms was demonstrated today, unfortunately I missed what must of been fun as I was on the 15:08 from London Bridge to Crowborough (Replacement bust to Uckfield from there at present).
As many will know the stock from the 15:08 also forms the 18:05 which is the busiest down train and is 8 cars. I get to London Bridge and the train appears to be 8 cars however it has two 2 cars at the rear. This is an issue because the units do not interconnect so south of Hurst Green passengers cannot get out the rear unit.
In the meantime someone tweets Southern asking if the 18:05 is 8 carriages, and Southern respond with a link to their website which doesn't list it. I therefore tweet that its 8 but actually only 6 are usuable. This puzzles the tweet team so I have to explain. After a while the 18:05 appears as a short form.
It must have caused confusion at London Bridge where the train would be advertised as 6 units, so people would be standing where 6 would usually be closest to the barriers. But it would come in as 8 with the 2 carriages nearest the barrier locked out! So a great shuffle would ensue!
I dont know if the problem could have been solved at Crowborough or the signals wouldnt permit it. But it could have come in to Platform 2 as 8, then split and the 4 rear taken up towards Eridge past the points, they could then come back in to Platform 1. The exercise could then have been repeated with the front and the two units joined back up. However the signals might not have permitted such a move.
This would have been a case of 2 171/7s (the 2-car ones) coupling up with a 171/8 (the 4-car one) at London Bridge before forming the 1508 down. When the guard walked down the platform they would have inevitably noticed the problem, maybe also once the doors were released. It is then a case of working out, as I have said above, when to lock out the rear 2 coaches. In this case I believe they must have been instructed that the whole formation would only run with the front 6 coaches in service at any one time. It would have been ideal to just detach the rear 171/2 and send it to Selhurst, but I don't think the driver diagramming at that point quite allows it.
It's not quite as bad as a similar problem I had the other day, which was a 12 coach 377 on the East Grinstead line with the London-end 4-car unit locked out of service as you couldn't walk through it. That did involve a mighty amount of platform shuffling!
As for the signalling at Crowborough, in theory I'm sure you could
physically do this, but it would not be within the scope of normal operations, and therefore each movement of each train would involve lengthy communications with the signaller, and in some cases they would also have to manually reset several items of signalling equipment which would have the potential to become very unhappy. Thus it would take a long time and block both platforms (each needed in tandem at the moment in the peak due to timetabling). It is also not now permitted to berth trains at Crowborough, IIRC, so you need to have a driver to remain with each separate train. If you wanted to then couple them up again, you couldn't do it anyway, as permissive working is not allowed between Hurst Green and Uckfield. Lastly, guards are not trained to uncouple 171s any more, so you'd need somebody else resourced to go through the detachment procedure with the driver. So... sorry, but no!