Great Western Coffee Shop

Although I am not a great lover of TWITTER, I was directed to this interesting discussion: https://mobile.twitter.com/25kV/status/1187468446516166657

Despite the negative views expressed, I can see the merits of discontinuous electrification.
One significant problem is the perceived need to lower and then raise the pantograph. This I would solve by making the overhead physically continuous, but with a dead portion under the low bridge. The dead portion needs no electrical clearance to the bridge structure.
Each lowering and raising of the pantograph carries the small but real risk of a fault or failure, eliminated by my proposal.

The train could easily coast for the modest distance under a bridge. No question of lowering the pantograph up to a mile in advance so as to allow a margin.

For longer problematic locations such as the sea wall at Dawlish, an auxiliary power source would be needed. I have long held the view that all new electric trains should have a diesel engine or a battery for when the wires come down, and for places like Dawlish.
For reasons given elsewhere, I don't think much of the IETs, however the inclusion of a single engine in the electric versions is IMO an excellent idea.

I have wondered that too, but  Post 36 onwards in Gary's discussion https://mobile.twitter.com/25kV/status/1187817168429371392

explains why it isn't possible to maintain the overhead wire in all but exceptional circumstances.  One obvious point is that many bridges are low enough that the pantograph would actually bash the bridge, as happened recently in East Anglia with one of the new bimodes when it went off-piste with the pantograph still raised. 

Isn't that what's already planned for some of the Valley Lines in South Wales?

I'm not sure how much extra height you'd gain from not needing electrical clearance?  Many of the bridges now have wires that are very close to the underside of the bridge (see the attached examples at Slough), with metal (insulating?) strips running over the wire for the extent of the bridge.  You still need some margin to take into account the effects of high winds and upward forces generated as the pantograph passes underneath, whilst still leaving room for the catenary wire above the contact wire.

Perhaps a bespoke dead conductor rod (like in the Severn Tunnel, but not containing electricity) might give more scope for providing less clearance, but how much would that cost to install and maintain?

Good question, and you're absolutely right to mention that scheme.  The trams will of course have batteries, and the much slower speeds mean that a lot of the issues discussed in the twitter thread are reduced. From what I recall, a lot of the gaps in wires will be more than just a bridge length too, which is more feasible with bi-mode operation. 

I believe a couple of bridges were so treated on the Paisley Canal line electrification, which was regarded to have been done "on the cheap".

There is provision in Series 1 for "in-running contenary", also known as “parallel contact wires”. That means literally running the contenary and contact wires side by side at the same height, supported by arms attached to the underside of the bridge. The contenary is a bit of contact wire spliced into the catenary, so it can withstand the sliding contact of the pantograph.  I can't see whether that's been used at Slough, and I've not heard anything about its use anywhere else. But it's in the manual, and it is the minimum possible low clearance: system height=0.

Correct, and has been used in other areas.

The technique of coasting through temporary dead sections has been used on the Western when the repairs have need to be done on an adjacent line, driver were warned and coasted the section.

The issue at Steventon is not so much and electrical clearance one but the rapid wire height transitions between the bridge and the level crosings

There has been a discussion of and/or link to that Twitter thread before. I can't remember where or when. I'd guess October last year and either the GWML electrification thread or the general technical electrification thread, but it could well have been somewhere else.

Having checked, contenary is routinely used under overbridges, where the catenary would have to be closer to the contact wire than the standard distance. That height difference doesn't always reduce to zero, but contenary is used for other reasons - for example, it's shaped so a registration arm can grip it. Going under the bridge calls for "bridge arms", and I can see some in the Slough pictures. I can also see what looks like parallel contact wires, on two of the tracks, but it's not that clear.

That series of tweets has added greatly to my knowledge on the subject, to a point where it will now register on a scale. I had never thought of the need to accommodate lightning. I am now convinced that the problem could not be solved by a neutral section or any other idea involving a train coasting through the section below the bridge. I now look forward to seeing what happens if emergency road works become necessary, for example a lorry strike on the parapet, something that will need much more careful planning with a 25KV supply running close below.

I agree with Broadgage that the inclusion of a contingency diesel engine was a very good idea, although I previously thought that he had lobbied for them to avoid interruptions to the power supply to the kitchen, should buffet cars finally return.

Contenary: a new word for me. Probably not one that will get a huge amount of use.

contenary is made out of 2 sections of contact wire.  The normal contact wire continues through at the height and stager, the catenary wire is transitioned from the aluminium / steel stranded conductor to contact wire.

Therefore the pan makes contact with 2 contact wires. The use of contenary maintains the cross sectional area of the overhead line.