Since I've been told off for using an existing thread, I'll start a new one. 1. This would be true, but bridges have interminable lifespans (120 years) so rarely come up for reconstruction at the right time (vide Lever Street on Manchester-Preston). Secondly, unless there is an active electrification plan, the Civils Asset Manager will never get funding to speculatively raise a bridge (unless it was cheaper to do it that way). Speculative wiring would only really be advantageous of each end of the new wire joined two places up (which might involve rebuilding, track lowering, etc. way beyond the bridge in question. 2. Some things are mandatory (when present): Providing vertical headroom for the OLE Finding somewhere to put the switchgear, feeders, etc. Providing the OLE Immunising the signalling system against 25kV induced voltages Protecting the signalling maintainer from the OLE (these last two sometimes trigger a resignalling of the line) Gauge clearing the route for the new electric rolling stock (generally fixing issues with platforms) Upgrading the earthing arrangements for station furniture, CIS, etc. and for fences and pipelines in the vicinity Raising parapets to comply with GL/RT1210 Things like speed increases and track renewal aren't a direct result of electrification, though advantage is often taken of the better acceleration curves of EMUs by doing so. Track renewal is typically a function of any speed increases and increased timetable frequencies. This latter is also the main trigger for closing footpath crossings, as it raises the risks, but the potential for fishing enthusiasts to zap themselves with carbon fibre rods has been another. 3. This depends on what performance you're wanting to buy. Current bi-modes are OK-ish EMUs with diesel donkey engines, good only for the smallest of gaps and useless on any kind of hill. If you went for a class 220 with a pantograph car added then perhaps this might perform better. Any change to and from OLE is best done stationary (to avoid embarrassment) so perhaps the IPEMU concept might be a better bet, as it doesn't involve starting diesel engines.