I imagine the duties the 755s carry out are not ideal for batteries. Long stretches on diesel only mean that a battery would not be so useful. Granted not a step backwards if the batteries are replacing a dead weight except for the cost of the batteries and the ongoing maintenance.
To draw a parallel the 231s have no batteries as they will eventually spend considerable amounts of time away from electrified routes. The 756s will take over from the 231s on the valleys when these routes are largely electrified - the batteries will only need to provide power for smaller un-electrified sections.
The batteries would be useful for regenerative braking, as well as for faster accelleration, no need to have diesel engines on at the terminals (trains seem to stand for 10-20 minutes at Lowestoft), and less diesel use as less of the energy needs to be supplied by the diesel engine and less idling. Granted, it is unlikely to be a massive reduction in diesel use, but similarly cost is unlikely to be high too
What maintenance would batteries require?
The 231s don't have batteries because they, like the 4-car 755s, have diesel engines in all 4 bays of the powerpack, so there is no space for a battery. I do still think tfw should have got 755s rather than 231s though, a pantograph would have been useful
Why would they need weighting? I'd always assumed the two engines were on opposite sides (and that's why they only come with two or four engines).
Can't be for adhesion because the motor bogies are under the cabs not the power pack.
Maybe
@dk1 knows? I just know that they're reported to have been weighted, no clue why it was done