Post by tut on Jun 22, 2017 18:18:14 GMT
Why is it that the doors of the front and rear cars do not open during this shuttle operation other than at Hammersmith (at least on the Eastbound wrong direction working, as those are the only journeys I have made so far)?
I thought that as long as the train was berthed in the right spot of the platform, the door operation would be unaffected no matter the direction of travel? Assuming it's a little more complex than that...!?
It is more complex than that, the equipment that provides CSDE (Correct Side Door Enable) and SDO (Selective Door Operation) information is only installed for the right direction. Or rather, it's installed for fully signalled movements. So at a platform like Edgware Road platform 3, where trains can arrive from both directions, the equipment is installed at both ends to allow for this. Obviously on the eastbound at Shepherd's Bush Market, trains generally only arrive in one direction, so the equipment is only at one end.
As a result the CSDE override procedure needs to be carried out. Now, as we know, the S stock is quite a lot longer than some of the platforms it stops at, so to get around that, ordinarily we have Selective Door Operation (SDO), where 1-3 doors in the front and/or rear carriages may be inhibited from opening if they are not alongside the platform when the train is stopped in the correct position. As such, only doors which are alongside the platform can open, and there's no danger of anyone ending up on the tracks. The information for SDO is provided by trackside equipment which is, like I say, only installed at the end that it would be needed at in normal circumstances. In some select locations this is both ends, but usually it's only one.
Because of this, when the CSDE override procedure needs to be used, the train doesn't know where it is and it doesn't know which doors it is safe to open, so it has to assume a worst case scenario and cut out all the doors in the front and rear cars, and then there's no chance of any doors opening where it wouldn't be safe for them to open. Note that this isn't just for wrong direction moves. If you overshoot the normal stopping mark you may end up out of range of the equipment and you'll need to use the override procedure. Again the front and rear doors will be cut out, even if all of your doors are actually alongside a platform, because the train doesn't know which doors it is safe to open because it's out of range of the equipment.
I should add, at Hammersmith, you actually do arrive in the right direction at the right end of the platform, so all the equipment is there and that's why the doors can be opened normally. The doors operate normally in the normal direction, it's only in the wrong direction doors need to be cut out.