Given they're basically AC and DC versions of the same design, why weren't the parts even more standardised between the 455 and 317? Surely it would have been a lot easier if things such as doors, windows, vents etc were actually identical?
Certainly shows a lack of quality control! I suppose this was before the advent and adoption of proper "Quality" systems.
I think it's more a case of York having one production line. If I recall the units were built in this order: -
Class 317/1 (317301-317348)
Class 455/8 (5801-5874)
Class 455/7 (5701-5743 - No TSO as they used a surplus Class 508 TSO)
Class 455/9 (5901-5920)
Class 317/2 (317349-317368)
Class 318
/2 (318250-318269)
Class 317/2 (317369-317372)
Class 318
/2 (318270)
As
O L Leigh says, the a.c. units were built as outer suburban trains, whilst the d.c. units were built as inner suburban trains (hence the low-back seating as used on the Class 313/314/315/507/508 units)
If you look at the order in which the units were built, then improvements actually came with each "batch", e.g. Class 455/8s had a more standard hopper ventilator compared to Class 317/1s; Class 455/7s had an improved front end over Class 455/8s; Class 455/9s had convection heating compared to pressure ventilation in Class 455/7s, plus a changed design of hopper ventilator and luggage racks (interior details). Thus design changes/improvements came with each batch.
This continued with the Class 319 units (greatly improved front end design, and increased seating in Driving Trailer due to new cab design with no separate cab vestibule) and then the Class 320/321/322 units that had no end doors. The interiors were also improved again here (i.e. much simpler ceiling/lighting design and the use of Ashbourne seating in Standard Class).
Now I believe that Class 317/2s were the first units to have the door windows the same height as the saloon windows, but that some of the doors for the first Class 317/2 units ended up on a couple of late build Class 455/9 units, either to trial them or in error. Parts would have been at York for both types of units, as they would have been on the continuous production line (i.e. 317/2s after 455/9s).
So it's nothing to do with quality control, but rather improving and updating a design. For example, the Airbus A320-200 has been in production for over 20 years now, but a 1990 built A320 will have a number of differences to a 2010 built A320, even though both are A320-200s! (Improved engines, newer interiors, improved avionics and electronics, etc). It still happens with trains today - SWT's original Class 450 units (450101-450110) were built with a different disabled toilet to that used in their second batch units, 450111-450127. Nothing to do with quality control, rather the phase two units have the same toilet as used in the Class 444 units, which is actually fully compliant with DDA regulations (something to do with sink not being reachable whilst sat on the toilet in the batch 1 units if I recall!).
