It is all well and good to talk of minimum numbers of units from a train manufacturer, but from the view of the train manufacturer, dependent upon the size of their manufacturing operations, they need to be building a certain number of units to keep operational costs at a commercially acceptable level, with production of other orders from different sources.
I wouldn't disagree.
Northern looking at replacing the 150's/769's and prospectively old "pacer" routes, I would be having a wager on some 20m stock being ordered.End gangways as per 150/2, and powered centre cars if they decide to have them in a 3/4 car formation.
With PRM,capacity is reduced considerably.a high density 150 can take about 140 passengers. PRM capable reduces that to about 110,which is a substantial drop.
not to mention,there are quite a few places that are limited to 2*20m vehicles.The pacers were a decent shoe-in for a budget route,so I would have thought northern will be wanting to keep their operating costs down.Sprinters are ok but given they are nearly 40 years old,there are definitely efficiencies to be had with better power plants and so on.
It doesn't have to be a 100mph rocket ship working on a branch line.Enough grunt to do a 0-60 repeatedly and back to 0 ,on a hard thrash duty cycle, and a few stints at 90 or so not to hold up mainline traffic is enough.
Some lightweight stock with easy splits/reconnects,and low running costs would work fine.