The big acceleration points are predictable. Electrify those areas. Then you only need enough batteries to maintain speed and get through the lesser acceleration points.
They would still want enough batteries to accelerate from a dead stop to prevent a 5 minute delay from becoming a 5 hour delay. Energizing the acceleration points would be more about reducing wear on the batteries and prolonging their life.
I don't think constantly raising and lowering the pantograph is a practical solution. Having lines with spotty overhead is very error prone and reliability is a big factor in rail. If we're talking about longer sections that are either electrified or not, things are different. E.g., extending the line for 100 km into an unelectrified part. Trains like the Stadler LINT (see my edit above), could make that happen.
> "I don't think constantly raising and lowering the pantograph is a practical solution."
I don't see why not. There are actually urban routes in London (London Overground) that do this multiple times on every journey! Switching between overhead AC electrification used in North London and the older third-rail DC electrification used in South London.
> "Having lines with spotty overhead is very error prone and reliability is a big factor in rail. "
But that's where batteries come in. Being able to keep moving under your own power if there's a problem with the overhead wires adds a lot of robustness/redundancy.
It depends on how long these sections are. We're probably of the same mind but thinking of different dimensions. If we're talking just for electrified sections of up to 2 km, alternating a dozen times or more, I'm not sure if it's worth it. For longer sections, yes, it's an absolute killer feature.
The cost of raising the panto slightly too early even once is tremendous. You would have to close the track for maintenance. So you need a safety margin of a few seconds. That's several 100 meters. Frequent changeovers would necessitate the system to be fully automated in order to factor out human error.
The changeover on London Overground is usually done while standing at a station (by the driver). According to [1] it's possible to switch while moving. The speed limit is given as 25mph, however.
Yes, you'd certainly make sure everything was automated or at least have a software lock-out to ensure the pantograph is only raised on the correct sections. But having a battery makes this a lot simpler, there'd be no need to stop or coast during the changeover.
It's a special operation, requiring special signage, and awareness of the driver. Here in Hamburg, where the S-Bahn using third-rail got extended to service Stade, is using multi-system rolling stock now. Which traverses the gaps by rolling without power through that gap. I also remember standing on some platform above ground, where you could see them coming over a ramp, with the pantograph up, on a piece of track using third rail. Wondering how that happened, and what would happen if it goes back into a tunnel (with third rail),
like that. Or under a bridge, overpass, all built with low clearance, because: hey, third rail!
Nerd that I am, I walked up to the driver, knocking on the glass and asked: "Moin, do you know your pantgraph is up?" Turned out he didn't, and there wasn't any indication. He got it down by repeatedly switching up/down, and finally it stayed down. Two minutes late!1!! ;-)
They would still want enough batteries to accelerate from a dead stop to prevent a 5 minute delay from becoming a 5 hour delay. Energizing the acceleration points would be more about reducing wear on the batteries and prolonging their life.