TS appear to think they have reasonable life span left in the decarb report at the end of July. They seemed quite clear they didn't want short term investmentFull electrification to INV and ABD must be several years from completion, perhaps more than ten (sorry don't recall whether dates have been published)? I'd be surprised if the HSTs could soldier on for that long, of if DR would want them to, when for their final few years a significant proportion of running would be under the wires. So I'd have thought bi-modes would be the way to go, at least for the interim.
There are comparatively few structures for clearance problems so not as involves as recent schemes e.g. GWML, MML, Edinburgh - Glasgow or NW Phase 1-4 & 6.