Welcome to another #RailwaysExplained thread… this is my first completely new thread on Mastodon, and boy is it going to be an epic one.
HOW IT WORKS (AND WHEN IT DOESN’T) – DISCONTINUOUS ELECTRIFICATION
Something I get asked about A LOT is #DiscontinuousElectrification. A typical comment on social media will be something like “well with battery technology so readily available we can just electrify the easy bits and run on battery in between…”
In this thread I explain why it isn’t that simple, and why proponents are often motivated by the wrong goals.
But first, we need to define the term and differentiate between different types of discontinuity.
Discontinuous electrification is a system where there is not a continuous live contact wire above the train. There are two distinct types of discontinuity:
1. Insertion of a Permanently Earthed Section (PES) of overhead line, with neutral sections & power switching either side of the PES, so that a contact wire remains above the train and the pantograph remains raised, but is not live;
2. Removal of OLE altogether, with OLE terminated & pantograph lower/raise operations either side of the gap.
We also need to address what the train is doing during these gaps - there are 2 options:
1. The train has switched to an alternate power source - typically either diesel, or battery - & will switch back after the gap. In the battery scenario, the battery is being charged in the energised sections;
2. The train has shut off power & is simply coasting through the gap, & will begin using electric power on the other side of the gap.
Additionally, we need to look at how this idea can be deployed.
There isn't a single way of doing this - instead we have a spectrum:
1. Continuous electrification
2. Continuous electrification with non-electrified diverging branches or sidings
3. One short discontinuity within an otherwise continuous system
4. Many short discontinuities, but still a mostly continuous system
5. Islands of electrification with multiple long discontinuities
6. Stationary overhead charging facilities at stations, with no OLE in between
7. No electrification
Options 1 and 3 are used throughout the world, & are just fine. Option 2 is increasingly being looked at for freight, with an electric locomotive with "last mile" battery capability.
Option 4 is about to be deployed for the first time in South Wales.
Options 5 & 6 have been used on some tram systems, usually because of aesthetic concerns about OLE, but to my knowledge hasn't yet been used on a mainline railway.
Option 7 - well, there's far too much of that, as anyone who follows me will know.
The final choice that we have to think of before we can discuss the technical pros & cons & the reasons people advocate for it, is that of permanence. Any discontinuous system can be either:
1. Temporary, as part of a rolling programme of electrification, or because a remodelling scheme is due to be undertaken in the gap. There is no point in electifying a railway that you are about to modify - electrification should be the last step
2. A permanent end state, with no plan to close the gap
This differentiation is key; option 1 is sensible - electrification takes time, and having an interim state allows the work to be broken down into manageable chunks that can be funded and minimise disruption to passengers and freight customers.
Option 2, however, is almost always proposed because "electrification of this location (usually a low bridge or tunnel) is too hard". The rest of this thread will focus on the idea of discontinuous electrification as a permanent state.
What do proponents mean when they say "too hard"? Electrification has existed for over 130 years; it is a mature technology, & every significant problem has been solved. If a tunnel is too small, track can be lowered or the tunnel rebored. If a bridge is too low, track can be lowered, or the bridge jacked or reconstructed. VCC also provides OLE solutions at many locations.
No, what they mean by "too hard" is "too expensive" - but expense is relative. Good value is sometimes initially expensive.
Railways have two sets of costs; capex (the cost of building it) and opex (the cost of operating and maintaining it). We all know that it is possible to make something cheaply and then end up paying a lot more in the long run - just ask anyone who owns an inkjet printer!
Furthermore, the overhead line is just one subsystem of the railway system. If we are to examine whole life cost (capex + opex) we must also consider all the other subsystems - including the train.
Capital costs for bi-mode rolling stock are very hard to come by, but industry sources I have spoken to suggest that a battery-electric bi-mode fleet will cost you twice what an equivalent pure electric fleet will.
Now lets take a whole system look at the concept - something that proponents often fail to do.
Since OLE is its own power transmission system, & power must be fed to the other side of the discontinuity, lineside feeder cables must be provided to bridge the gap.
We can't leave the power changeover process in the hands of the driver; especially with multiple gaps, mistakes will be made with dire consequences.
So power changeover is triggered by lineside balises, adding both lineside & rolling stock complexity & cost.
We'll talk more on these aspects down thread...
Recent experience with operation of bi-mode trains in the UK shows that there are significant operational and infrastructure constraints on the placement of changeover zones, to avoid driver overload & minimise the risk of a dewirement due to raising the pantograph in a complex area.
This makes it very challenging to have multiple discontinuities on all but low-speed routes.
Let's dig into that challenge in a bit more detail...
At this point you might be thinking "just use gravity? Have your gap on a gradient. Take off the brakes and roll out." Unfortunately you still need to electrify in the uphill direction, and so you still need to rebuild the problem bridge. So you might as well electrify both lines.
But lets assume that you've found a location where you can coast without risk of coming to rest. Hurrah! Take your £2m bridge recon cost and spend it on party hats.
Not so fast roadrunner...
Before you can start coasting you need to lower the pantograph. Easy enough. But what happens if the driver forgets? One smashed pan, one smashed bridge, one disrupted railway.
We don't tolerate single point human error failures like this - we build systems that prevent the mistake from having a consequence. In the UK this means trackside balises or RFID transmitters that lower the pan automatically, using some sort of Automatic Power Changeover - APCo.
APCo can only be used by ECTS-compatible or otherwise bespoke stock. So you are locking in a specific set of stock, and locking out operators with older cascaded stock, or even the TOC wanting to beef up service frequency.
As for freight, forget it. It is possible for passenger trains to have a battery big enough to power through multiple gaps, but freight requires several times more power. Any battery big enough will add weight, thus requiring more energy, and a bigger battery... and so on.
The process is complex. A control balise tells the train to lower the pantograph; but as a failsafe a further zero balise is placed beyond the lower zone as the last line of defence; it tells the driver to lower the pan or stop the train.
If the pantograph fails to lower, the driver must bring the train to a stand to avoid damage. So the zero balise must be at least the braking distance before OLE end.
The cumulative effect of this process is that at 125mph the zone is ~ 1.2km long. #ALT4me
With all this going on, the changeover zone must be carefully selected so that the driver is not overloaded with other information or duties. So no signals, stations or junctions.
This can push the changeover zone a long way back from the actual end of OLE - for a while the GW one was East of Didcot, several miles from the actual OLE end which was West of Didcot; because the junctions and station would have resulted in driver overload.
Of course on the other side of the gap you need to raise the pan again - you can't do this just anywhere, you need to select a spot where the OLE is boring so dewirement risk is minimised. So again, no junctions or stations.
But lets assume that you've found a location where you think you can fit all of this in. Hurrah! Time to take your £2m bridge recon cost and spend it on party hats.
Actually... we're still not done.
This means you need to run HV insulated cables in trough route along each side of the railway, and connect them to the OLE at each end of the gap. This isn't particularly cheap: I've heard figures of £500 per metre quoted.
All of this equipment - APCo, balises, power feeds - will cumulatively erode the savings that you make by not rebuilding the bridge. But lets pull back from the detail and look at it from a much simpler point of view.
Discontinuous OLE is appealing - it looks like a great money-saving idea, right up until the first time it doesn't work; then you have a stranded train, a rip down, a pan smashed on a bridge, or all 3.
Your gap will be a permanent operational headache that requires an expensive fix.
So multiple discontinuity electrification swaps the high capital costs of enabling works at low bridges and tunnels for increased capex & opex elsewhere. These costs include:
- Increased rolling stock capex and opex;
- Increased cabling and lineside comms capex.
Interestingly, recent developments with Voltage Controlled Clearances (https://mas.to/@25kV/110866820613809092) mean it is possible to use the same amount of space that a PES needs to provide a continuously energised system instead.
Attached: 1 image Another #RailwaysExplained thread... this one is another port over from the Site We Don't Mention. More to come! HOW IT WORKS - SPARK GAPS AND SURGE ARRESTERS One of the things that I spent a lot of time on back in 2019 was sourcing good quality images for the #OLEbook - getting the more obscure stuff from publicly accessible places was quite hard. That's why I had to go all the way to North East England to hunt down this Spark Gap on the Tyne & Wear Metro #railways #OLE #OCS #OverheadLine
But there are wider economic issues that must be considered as well. A national railway system is just that - a *system*. Discontinuous electrification erodes one of the basic economic tenets of that system - that is, the ability to repurpose electric rolling stock around a network built on common standards, as traffic patterns change and train fleets age.
This cascade process, where a new fleet displaces a mid-life fleet to a secondary route, is at the core of railway fleet economics.
@25kV I mean the no overhead and coast through, combined with maximum pantgraph height blocking on the train and release/catch section to guide it on the OHLE was the default in The Netherlands for a century or more for bridges. Now with reconstruction they do try to add OHLE on the bridge if at all possible as it takes a risk out of the network, but it is still not mandatory.
And yes, has to be taken in neutral, I've seen the arcs if that fails (just increases wear, but looks specacular
@25kV Well if it's clear from the very start that you'd make a good use of short discontinuities, the OLE design and pantograph mechanical can accommodate that as in this 1.5kV territory:
- https://youtube.com/watch?v=yoCKS4IkzXY&t=2470s
- https://youtube.com/watch?v=GBV3St0hhgY&t=29s
I can imagine the OLE geometry part however how the pantograph can stick at the same height without springing up is beyond me.
@25kV Granted, I come from a US freight perspective, so I understand you'd give up the advantages of a married set and have the added labor of the hostling...
Another way of asking is "how much capital in battery refurbishment could be saved by charging batteries in an ideal way?" Would the cost of having more tenders (at least one charging, and one in service) balance with a reduced cost of a simpler locomotive?
Would route flexibility extend the practical life of a trainset be worth it?
This made me wonder whether there are any diesel bimodal trains where the diesel engine is a separate car (i.e. there's a separate car that contains the diesel engine and generator but without traction motors), so that you can do swap them out and have unsynchronized maintenance windows on them. Do you know of any such?
@robryk Yes I do - they're called locomotives.
Seriously though, modern multiple unit trains are computers on wheels; swapping out a vehicle requires complete reconfiguration of the software for the whole train, so isn't something done lightly.
If you want swappable tractions units, buy locos and carriages 🙂
@25kV Sorry, I wasn't clear enough. I meant a car that provides electrical power but no traction, so that an electrical locomotive can provide traction.
But fair point re swaps being time-consuming, so this probably makes such an approach useless.
@25kV It's not just the monetary issue of rebuilding bridges and tunnels - it is also the disruption that it can cause. Take the electrification of the Bolton-Manchester line, for example. The bridge at Moses Gate caused disruption for months as it was a major thoroughfare for the town's car traffic, contained utilities and the geology underneath wasn't as expected.
If they could have run on a battery through this, it would have saved so much time and disruption.
Garry Keenor
Helle, Queen of Dissociation 
David Jaša
Garrett Wollman
BackAlleyUrbanist
robryk
arthur