For NR the first question is whether it's practical to install just standard 25kV AC. That might require special treatment on the bridge with custom kit, but if that can provide a consistent 25kV AC for trains to use in the standard loading gauge, then there's no need for special trains at all.
If that isn't practically possible, then NR has to come up with a system which isn't only applicable to the Forth Bridge, as there are huge numbers of other places on the network with limitations on OHLE install. Consider all of the tunnels, low bridges, sensitive areas, depots, level crossings, dual electrification zones and the like across the network. Come up with a solution for all of them and you make it more worthwhile and affordable to fit it to trains, and make it possible to deliver the benefits of electrification at a lower cost on more lines across the country. Would 3kV DC/AC help? Not really - there are still plenty of situations where the existence of an electrified overhead line at all is a problem regardless of voltage. Would narrower pantographs help? Again, not really. It'd be marginally helpful in a few low-clearance situations with rounded corners but it won't do much elsewhere.
Small batteries are the only technology which would be useful and applicable across the entire network. It'd be massively helpful for NR if electric trains were able to rescue themselves to the next station or off the mainline when power supplies fail. For DC trains it would help capture regenerative braking energy when it'd otherwise go to resistor grids at the substations. It would allow DC and AC supplies to be separated when there's a switchover. At stations it would massively reduce the cost of electrification as the platform area could be made a neutral section, meaning that nearby bridges which were acceptable under previous standards will no longer need raised or replaced.
