The problem is that it's not double track which has been singled - it's just a single track formation. Realistically it's never actually going to get doubled.I don’t know the line but on the single track section could you not install twin track cantilevers such that they wouldn’t need moving, and just position the equipment on them accordingly in the interim (presumably around the mid point)?
Buying BEMUs and dragging batteries around all day every day for years isn't without cost. It makes no sense to do it just because otherwise you might one day want to double a line, and you'll therefore have to resite some electrification masts when you are rebuilding entire embankments.