I think the advantages of a D78 conversion over a Pacer are bogies, sliding doors and better transmission of power from engine to rail. I will wait to see how the conversion actually pans out. I think the challenge will be to mount a heavy lump under a coach not originally designed to take it (or were they ?).
I think the beefing up of the cab end seems to preclude the chance of having an inter-unit gangway.
A thought I had was, assuming both bogies on a D78 driving coach are motored, why not move one motored bogie to underneith the intermediate trailers along with one of the two power packs. That would give a better spread of weight but of course is irrelevant if the conversions are to only be 2 car units. I am assuming that moving a motored bogie from one coach to the next is possible (as in similar enough mountings and space).
Question I have (prompted by the above idea) is will the power derived from a pair of power packs match the capability of the existing traction motors under one coach or be more/less. So could half the traction motors under one coach handle the power of two power packs ?. Better still two lower powered packs under each coach would be more reliable as less stress on body and engine. Could also be viewed as less of a design risk but more cost due to extra power packs.
Obviously older trains/designs are not going to be good at coping with an intensely used railway line. In the mornings at East Croydon I see mostly twelve coach trains (2*442s are equivalent to 12 car 377s) going through platforms one and two nose-to-tail towards London and leaving passengers behind (me !). Contrast that with the routes where the converted D78s are expected to be used.
I do fear that, like the pacers, the D78s will end up being used for routes they were not originally intended for. Were they not originally intended for rural branch lines but their bogie-less wheelbase made life too difficult ?. I hate the Pacers but they were something cheap enough to be affordable for lines that could not justify higher expenditure. However that seems to be where we are again now and something is better than nothing. The alternative is to put the fares up in the hope of being able to afford newer trains and/or reduce demand.
I would, of course, rather the wires were put up and old ELECTRIC trains were used - but that is not happening fast enough. We will get there (mostly) eventually but we have to accept that the current electrification projects will be followed by yet another long pause - history repeats itself. We have had three bursts already:-
Early-mid 60s West Coast Main Line (last bit in the mid 70s),
15 year gap,
Mid 80s East Coast Main Line,
20 year gap,
Currently embarking on GWML and MML plus the North West.
I am missing a few things but basically we have more routes being electrified now BUT after a longer gap. More time spent NOT electrifying than actually doing what Europe finished back in the mid 60 !

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