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Of course that assumes the 377/6 replaces the 456 completely where as they don't actually.
Note as well during fleet introduction you don't just replace unit for units initially but allow for a build up of units in case there is an issue. Called sensible planning.
It does seem to be the case that the 377/6s are not directly replacing the 456s. So it is not possible to compare coach for coach.
Currently, from what I have seen, the 377/6s are being used in pairs to replace 377s that are formed 3+3+4 car. The 3 car 377s then seem to replace 456s. I am going to assume that 3+3 car 377s replace 2*456s (2+2 car). So one pair of 377s (10 coaches) INDIRECTLY replace one pair of 456s (4 coaches). The other 6 coaches are a 4car 377 going somewhere and 2 car lengthening. So that means that the 24 456s are replaced by 24 377/6s.
The above is a bit of a wild guess I confess. But the whole point of the 377/6s is to facilitate extension of the 8 car metro routes to 10 car is it not ?.
The 377 services that have been formed 3+3+4 car were not always so. They used to be 455+455 or often 455+456+456. Six car trains were 455+456 or quite often 456+456+456. Showing my ignorance here but how long have there been 10-car services ? These seem to be 377 services (3+3+4 car) ?. Last time I was paying attention I only saw the 377/3s working on their own and sometimes as 12 car services !. Certainly not mixed with 4-car 377s on Metro services.
I also notice some other changes to diagramming seem to mean that Southern are not having to diagram so tightly. Perhaps to make use of the extra stock they have. I was thinking of the following post which I have now dug out.
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I was interested to see 377/6s on the 04.56 Balham to London Victoria and wondered what was previously allocated. The units on this used to go empty from Victoria to Dorking but now the 0659 from Dorking appears to be formed of units which spend the night at Dorking working down empty from Selhurst the night before -
http://www.realtimetrains.co.uk/train/W77174/2013/10/29/advanced.
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So, we have even more likely confusion over first class on the 377/6s than the rest of the 377 fleet! No yellow band, combined with a lack of "1" stickers sounds like a planning catastrophe - may people don't even seem to know that first class is first class on the existing fleet, judging by the numbers of standard class ticket holders that use them!
Regarding First Class. In my experiance on existing units its only the headrest cover that makes me think perhaps I am in the wrong place (being a tight fisted SECOND Class traveller). The yellow line outside is only of use to those looking for their First class seats. I think it is too early to tell what the final first class marking will be. Only a week ago there was no indication or need to. Its only appeared since the 377/6s started on the East Grinstead services.
I do notice that the seats etc in First class are nothing special but imho the reason people pay for first class is to get a seat rather than risk standing. Ignoring intercity. Twenty years ago the season ticket for First class was not that much more expensive than the Second class season ticket (South Central East Croydon to London).