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 OLE, with neutral sections and power switching either side of the PES, so that a contact wire remains above the train, but not live;
2. Removal of OLE altogether, with OLE terminated and 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.

But first, we need to look at the ways 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 discussed 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, and every significant problem has been solved. If a tunnel is too small, the track can be lowered or the tunnel rebored. If the bridge is too low, the 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" - which is a very different reason.

Even then, railways have too 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 more in the long run - just ask anyone who owned a car in the 1970s!

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.

Bi-mode trains are much more expensive to buy than pure electric trains. If the 2nd mode is diesel, you have all of the disadvantages of that; lots of moving parts, higher maintenance costs, more trains out for maintenance so a bigger fleet needed. If battery, you have to replace the batteries within the working life of the unit - railways need deep cycling fast charging, which is hard on battery chemistry - adding more cost.

@25kV

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?