An interesting discussion, and thanks to all who have contributed. I'll come back to the original question, is there any reason why equipping emus with a small battery pack which provides resilience in the event of a stop on a neutral section and which also enables zippier acceleration (which would be useful given the number of stops on valley services) wouldn't work or be cost effective when compared with the alternative of rebuilding numerous bridges?
There are two parts to your question.
1) Could you give an
EMU▸ a battery pack and/or diesel generator to allow it to work when the wires are off? Absolutely. The battery pack concept is newer on heavy rail, but there are a number of tramway systems that use it, Birmingham's have passive-provision and will be retro-fitted for when the next extension is complete. The all-electric version of the Hitachi
IET▸ has a small diesel engine for if the wires go down, though it's really just enough to keep the lights and A/C on whilst it limps to the next station. The new conversion of the 319's has a diesel engine that effectively pretends to be a 750v input from a third-rail, so if the pan is down, the engine feeds the 750v bus. If you've got a 25kV supply you'll have all the zip you need, but batteries could be helpful to improve the acceleration of a
DEMU▸ .
2) Can you get away without rebuilding structures? Yes and no. Generally the pantograph has to stay up (albeit that it can be pretty low), so you have to have a mechanical clearance for that. It's generally not the height of a structure that is a problem but the 'shoulder' so you don't foul the horns of the pantograph. You then have the electrical clearance between the pantograph,
OLE▸ and structures, which can be surprisingly small, particularly if insulated sections or rigid catenery is used. In certain circumstances the wires can be really low, for example on the Paisley Canal branch where low-roof rolling stock is used, but this means that stock like 66's can only be used with the OLE isolated. The clearances between humans and OLE have to be pretty large under the new rules however, hence all the wall raising and footbridge replacements, though there is scope for derogation with the right paperwork, for instance on some of the newly electrified platforms in Paddington.
As for new third rail, forget it. The only time that will ever be allowed is in locations where access is highly restricted e.g. tunnels, depots etc.
From what I can remember, there should be plenty of space under the Students Union in Cathays, though I suspect the footbridge needs to go, but to be honest, a
DDA» compliant footbridge with lifts would probably be a pretty good idea anyway. And therein lies an interesting point. Much of the upgrades that accompany electrification are stuff that really ought to be done anyway - like re-signalling
