An additional intermediate trailer vehicle per set will not be particularly expensive compared to the power cars and being fairly lightweight will not impact significantly on performance. The new fleet acquisition might be a one-off opportunity to also purchase these to cater for future traffic growth which I believe will occur on IOW once the quality of service goes up. The extra trailers might be taken out of the sets and stored during the winter to save some energy carting them around. Perhaps Vivarail with their whizzy American backing could organsise some clever leasing deal so the extra cars are not paid for while they're not being used.
you could say the same about quite a lot of rural lines where there is "peaky" demand seasonally.
a modular 15-18m 2/3 car hybrid sprinter replacement fleet would be ideal.(i'm thinking articulated bo-2-bo or bo-2-2-bo configuration)
2 cars(153 replacements) can be light(around 25-30t per car compared to 40 for a 153 or 35 each for a 150), low track access charges and PRM compliant, and giving actually a bit more flexibilty than a 153 as they will still have 2 engines and limp home on 1 if need be.not to mention a 15 or 18m car will have less trouble with clearance on some nasty curves on the network.
for the purposes they would be required for 1*300bhp per car keeps the fuel costs down and will still permit up to 90mph running(pacers are 25t and can do 75mph on a 250bhp block...2 powered axles per car also helps with adhesion and 2 axles per bogie gives much better comfort/less squealing over bumps and curves)
3 car is basically go to workshop,drop in an extra trailing bogie and the fuselage, then connect electrics and air/vacuum for brakes.
could be done overnight or when refuelled/serviced.