Would the extra 140 vehicles planned for SWT allow this to happen? If the extra 6 sets created from the 458/460 plan were in place would this allow SWT to give up some of its 158 fleet?
The SWT 140 vehicles was intended to be 140 EXTRAs, and all EMU carriages. The HLOS proposal was for all inner suburban services to be 10 car, and for all outer suburban services to run at maximum length, ie bring some 8 car servcies up to 12 car. None was intended to allow DMUs to be cascaded.
Just expanding the 455 fleet somehow so that all could be diagrammed as 10 car would be 90 odd carriages. Another 30 would cover making the HC 450s into 5 cars, with about 20 cars left over, say 4 or 5 extra EMUs.
The size of the 158 fleet was determined by the need to increase more Salisbury - Waterloo services from 9 to 10 car, and more 6 cars up to 9 car. 10 car DMUs would run as 2.159+2.158 - there is one such service each morning already, if there were three such trains that would need 9 of the 11 158s in service, including the 3 on the Salisbury - Romsey service.
As we've determined in the past, if a few more EMUs became available, a few 158s or 159s might theoretically no longer be needed on Hants locals over the third rail, or on the Lymington branch but maybe they'd just get used to strengthen services on the Exeter line, not necessarily be cascaded elsewhere.
Using 2 x 5 car bimode IEP to Salisbury with half continuing to Exeter would be a probable solution once the 159s are retired, I guesss that could be 15 - 20 years time? Alternately if IEPs were ordered soon the DMU fleet could be cascaded for about a 10-15 year life on regional routes. Would this be economical? But IEP, (just like 180s), may be over-speccedfor the route - it isn't exactly a fast line.
Whatever, it certainly wouldn't be a decision for SWT to make.
