If additional class 158's become available to allow SWT▸ to extend their Salisbury - Romsey - Southampton - Eastleigh - Romsey service to Swindon then SWT should be pushed to do so.
If emu's become available to do so then SWT should make use of electric traction on the Lymington branch releasing that 158 to work around the Salisbury area.
It's not as easy as that is it? A
TOC▸ just decides to run a new service across the neighbouring TOC's area? Revenue abstraction problems?
FGW▸ would normally expect to run any new service in their own area.
Paul