Instead of being a threat, Bi-modes could be seen as an opportunity to carry out a rolling programme of progressive electrification, without the pressure of a fleet renewal deadline.
Even while still carrying their diesel gen sets, every mile saved on diesel is engine hours and fuel saved, with maintenance and off wire range implications, so gradual further extension of electrification can still makes sense if infrastructure costs can be brought under control. As the proportion of off wire running in typical diagrams decreases, the practicality of replacing many if not all of the bi-modes' diesel power packs with moderately sized battery packs increases, so they could continue to be able to run through short gaps in wiring where provision would be very difficult or expensive. Pennine tunnels are an example. The alchemy will then be in optimising the size of the on board batteries (and any remaing self powered engine or fuel cell modules) to the characteristics of the diagrams served. The modular nature of the AT300 trains seems to lend itself to this kind of evolution over its service life.
There's no problem starting and getting up to linespeed under a moderately sized battery, just in repeating that exercise many times and sustaining prolonged high speed running. You could get to the point, say where only 10% of a route remained unwired, when battery storage alone could suffice to cover the gaps. What I'm suggesting is that while in the process of achieving that, operators could still save fuel and engine hours by exploiting new sections of wiring as they are gradually energised.
To me the Cardiff Valleys electrification is possibly the most interesting electrification project underway in the UK at the moment. The use of OHLE/battery trains there doesn't preclude further electrification on the NR controlled infrastructure that the tram-trains and FLIRTs will use. I hope that once the new Valleys team has demonstrated they can plan and implement cost effective electrification, they will be let loose by NR and TfW on the VOG, Maesteg, Ebbw Vale routes, the main line to Swansea...and beyond. The big prize possible with Swansea, along with Oxford and Bristol electrification, would be to allow the gen sets on a large proportion of the GWR IET fleet to be removed. A similar approach could apply on the MML. I would say that within the life of the fleet, there's a good chance of that happening with some future enlightened thinking. As others have said the problem is not the intrinsic expense of electrification, but that of a particular difficult project that must never be allowed to become sole the benchmark for all future projects. Bi-modes provide the interim pragmatic measure to enable the electrification to proceed further without all the dependencies of the initial scheme, and when new wires are energised anywhere en route it is always the time to raise the pantographs and switch off the diesels through the section, as this will save (somebody) some money and probably improve performance. When under the wires there is clearly a weight disadvantage in having to lug the diesels and fuel tanks or batteries around, but that doesn't seem to impact much on the cl. 80x acceleration which is stunning in my (admittedly limited) experience, as the OHLE supply is able to reliably deliver the power required, and once the train up to line speed then the extra weight has little effect on the energy used for cruising, especially on level track.
On MML, much of the signalling is now OHLE ready. Once the new fleet is in service, a steady programme could proceed in the background to adapt bridge and station clearances, and could allow a staged introduction of more OHLE on a much more relaxed timescale.
