It is said that London Midland have the option to order an additional twenty six class 172 vehicles as a follow on to the current order, which I envisage as an additional four 3-car units (172346-349) and seven 2-car units (172223-229). I only hope that this option is taken up before the offer lapses and the opportunity is missed.
With that many additional 172s, they could cover the Chase Line services and the Marston Vale line (with some tinkering with platform lengths. particularly on the Marston Vale line), allowing more of the Shrewsbury services to be formed of pairs of 170/5s, released from the Chase line, as you desire and as would generally be sensible.
So two of the 7 two car units could be used on the Marston Vale Line (once platforms have been extended), one on the Coventry - Nuneaton Line, four of them could go to the Chase Line (forming two four car 172's, once platforms have been extended), thus releasing four 170/5's to go to the Shrewsbury Line.
All of the 3 car units could go to the Snow Hill lines, with the 2 170/6' being sent to the Chase Line and providing a net increase of one 3 car unit on the Snow Hill Lines.
So that would see the Chase being opperated by two 4 car 172's and two 170/6's.
The Shrewsbury Line would be opperated by 3 four car 170's during the day and a further 2 four car 170's would be available for peak extras.
So that would mean that all 6 170'6's would be used all day (four on the Hereford Line and two in the Chase), and ten 170/5's would be used on the Shrewsbury line (six in the day and four for peak extras).
Asuming the in a fleet of 23 trains, three of them would be needed for maintenance and emergency cover at any one time, that would leave four 170/5's to work the pea extras on the Hereford line.
So ordering those extra 172's and extending Chase Line and Marston Vale platforms, probably would be the answer to capacity problems in the medium term at least.
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And that would also allow all 153's to be casscadded to other franchises.