Plus how do you justify extending every service to 4x23m coaches when Northern already require a large subsidy? No, a "one size fits all" arrangement would never work.
You don't/can't.
What you can do though is order a mixture of 2, 3 and 4 car DMUs.
The 4 car DMUs would be more cost-effective than joining up pairs of 2 car DMUs and keeping them joined up all day, which happens on some services like Southport to Manchester Airport.
Having a mixture of 2, 3 and 4 would also allow overcrowded peak time 4 car 150s (like on the Buxton line) to be replaced by 5 car workings, which is less likely to mean an overcrowded service suddenly has a lot of spare unused capacity that might occur with a 6 car working replacing a 4 car working. Also there are some platforms which can take 115m trains but not 138m trains so it would also be more cost effective (at least in the short term) from less funds for infrastructure improvements.
One thing that's not come up in this thread yet is tram-trains. The low-floor ones being ordered for the Sheffield-Rotherham trial can't be used in the Manchester area. While those ones will be electric, a lot of the potential tram-train proposals will be for diesel or bi-mode tram-trains due to both the cost of electrification and the issues with DC OHEs on the National Rail network.
nobody wants 3+2 seating but then they complain about crowding if they have to stand.
Add to that no-one really wants to have to pay extra and buy a 'First Class' ticket just to sit in a 2+2 seating compartment on a commuter train when other commuter routes get 2+2 seating in standard class.
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Like the North Berwick branch, these lines got cascaded slam door EMUs from the London area (305s to North Berwick, shortened 308s in Yorkshire).
I hope that others bear this in mind when they complain about newly electrified routes in CP5 getting 319s etc.
The cascaded 319s by 2014 replaced a previous government plan for an order of brand new DMU vehicles to be introduced during 2012. At the time we were reassured that the 319s would be completely refurbished before being introduced in the North West. Now it doesn't sound unlikely that they'll be introduced in their current state and there may not even be the required number coming up in time for the December 2014 timetable change. So we may have gone from new trains in 2012 to cascaded refurbished trains in 2014 to possibly cascaded unrefurbished trains not before 2014.
Although I understand that Yorkshire was all set to get new class 323s by around 1994 but Metro had funding issues following central government cutting back PTE support (which led to cut backs in other areas as well like Sunday services in Greater Manchester being cut back) and so the order was delayed and cascaded 308s were taken on. However, a lack of train orders meant that Hunslet collapsed and Metro had to look elsewhere when funds became available.