Hi, I'm working on a fiction project set in a rebuilding society 100 years out. It's fairly utopian so people are prioritizing restoring and improving train infrastructure and service pretty significantly. American cities have high speed rail connecting them, and because I'm building this story out of my daydreams, defunct short lines have been returned to service and even rural towns have some kind of passenger train running again.
The opening has the players traveling way out into the boonies, and part of setting up that 'traveling off the edge of the world' feel was having them leave whatever city they begin in by HSR, then take successively cruder public transit until the line just ends short of their destination and they have to figure the rest out on their own.
The second-to-last step in that chain is an electric self-propelled railcar traveling along a restored short line. I think the current doodlebug is a somewhat-recent thing, put together maybe within the last ten years, and probably planned to be temporary when they built it.
So my question now is: how would they build this thing? Would it be easier to convert an existing self-propelled railcar like the Budd RDC? Would it make more sense to start with a regular coach and retrofit in the batteries, motors, control station, pantographs etc?
Or would it make more sense to start from scratch? Maybe use part of an electric bus or similar?
Reuse and salvage play a pretty big role in the story, so I'll probably embellish any option with details about where the parts came from originally.
Thank you for any advice!
Do I understand the inquiry is for how to build a line-powered electric, single railcar by retrofitting? In terms of engineering complexity, starting with an EMU and cutting it down to a single railcar would work.
If not that, then a battery electric railcar would work as a base, since it would only need the line power equipment (eg pantograph, trolley pole) added, and disabling/removing the battery pack.
If not that, then a diesel railcar -- or DMU and cut it down -- and swap the diesel generator for line power equipment. The criteria is that for any self-propelled vehicle -- rail or otherwise -- having to reconstruct the propulsion mechanism is a big ask.
For that reason, the conversion of an unpowered railcar -- like a passenger coach or a freight wagon -- is way down the list, as any existing vehicle with propulsion makes for a better starting candidate.
So well before that, we would look to putting other land vehicles onto the rails. A motorcoach bus is a good candidate, but if hauling unpowered wagons is allowed again, a tractor-trailer cab (aka 18-wheeler) could be electrified and then tasked with pulling a consist of trailers converted with passenger seating.
I wish to reiterate that the effort to add a drivetrain to an unpowered vehicle is very high. Some vehicles might not even make this possible: imagine starting with a wellcar. There wouldn't be any room to put the drive motors near the bogies, without cutting and modifying the frame. And then it would need an operator cab, overhead power equipment, all manner of electric wiring, and so on. And that would still only yield a freight self-propelled railcar. More work would be needed to bring this into passenger service, unless the passengers are fine riding on an open car.
This is really good information, thank you for explaining where the complexity really starts! I especially appreciate the heirachy of practicality. This is exactly what I was asking. So far it seems like there is a much wider range of options than I expected so I'll think on what best fits the setting.
Thanks again!
One other thing I wanted to mention is the complexity of supplying power to overhead lines. This is based off a thought experiment I had long ago, about whether a municipality could "grow into" an electric rail network, by initially underprovisioning the power system until more trains in service demanded that upgrade. My conclusion was that no, it doesn't quite work like that. These are my observations.
The primary issue is one of raw power. That is, delivering kilowatts over wires that could be very long. We can consider Denver's commute train operation, which specifies mainline speeds (>79 MPH), 25 kV AC power, and an output power of 620 HP (456 kW).
That 456 kW is the focus, since that's generally what's needed at either peak acceleration or at top speed. We should assume that overhead lines should not wreck themselves just because a train is running hard.
If there's only one train on an overhead line segment, then the power requires is the same as one train's draw, which is 456 kW here. The problem is that compared to what's typically provisioned for a home (200 Amp, 240v service; aka 48 kW) or a light commercial business (400 Amp, 120/208Y service, aka 144 kW), this is a massive amount of power.
And even when a power company does supply a neighborhood of homes or a commercial district, they use oil-filled transformers that can be intentionally overloaded by some 30-50% for hours, on the premise that peak electric loads would die down afterwards and the transformer can cool down. Also, not every home or business uses anywhere near full power, so transformers are also undersized accordingly.
But for a single train, the need for that 456 kW is very real, very present, and if the overhead line is even 10 miles, that's 8 minutes if the train passes through at 79 MPH. If only one train passes per hour, the average power is only 60 kW but I don't think any transformer rated for 60 kW could survive an overload of 456 kW for 8 minutes and cool down for the next 52 minutes. That oil will have boiled by then.
So in reality, to provision power for just one train per hour, the transformer has to be rated for something closer to 200 kW. An entire suburban subdivision might total up to 200 kW depending on the time of day, and somehow the power company would need to supply this to wherever the railroad's power conversion equipment is.
So in your story where different communities are working to rebuild tracks and electrify, the latter effort has some gargantuan hurdles. The nature of the electricity network precludes attaching a 200 kW transformer to any random point in the network. A residential neighborhood might be fed with a 7200v 600 Amp ring circuit, which also connects to adjacent neighborhoods. Attaching the transformer to this circuit would work, but it would singlehandedly be 5% of the ring capacity. And the ring has to be nearby the railroad's power equipment. So stringing new high-voltage power lines toward the railroad is highly likely.
And this plan isn't even great, because the train passing would cause a lot of issues for the neighborhoods' electricity voltage stability. There's also the problem of supplying 7200v (called "low voltage" in the industry) if the overhead lines are meant to be 25 kV. Converting voltage up at the consumer point is generally not a good idea for efficiency, so a realistic rail power system would need to attach to medium voltage (eg 36 kV) or high voltage wires. So now our transformer needs to be located somewhere near such wires, and also will cost more because of the high voltage rating. High voltage wires are only placed where they are by necessity, because they're awfully dangerous otherwise. Some communities may be miles away from the high voltage lines that eventually power their homes.
In terms of technical requirements, this is rapidly getting out of hand, and I cannot see how a community smaller than say 50k-100k people would have the electricity resources and knowledge to build out an electric rail supply. And it only goes up as this piece of track gets more trains per hour.
If your story does wish to hew towards the almost-insane engineering for electric rail systems, it might be worth examining how Caltrain in the California Bay Area electrified their 50 mile corridor, between Silicon Valley and San Francisco. IIRC, they needed 10 power transforming stations along the route, with special engineering for each one of them.
Overall, this is why rural areas (and even semi-urban) don't tend to have electrified rail, despite having electricity service for streetlights, homes, and retail. Because they really just can't do it.
Does the math change at all if they're only trying to power a single electric bus converted to rail use? I'd planned on some kind of single vehicle, but I'm not sure what factors lead to such a significant draw.
Thanks!
The two major factors for vehicle energy consumption -- rail or otherwise -- are rolling resistance and air resistance. For rolling resistance, this primarily means the weight of the vehicle, because the rolling resistance is proportional to the force on the wheel bearings. And that goes up with weight.
So if you had a 40,000 lbs bus versus a (very light) 40,000 LRT car, then the rolling resistance will be similar. That said, the next biggest factor is rubber tires vs steel wheels. But since this is a converted bus for use on rails, that becomes very similar. The third factor is the number of wheels that the weight is spread over. A bus might only need four wheels touching the rails. But an LRT car might have two bogies, so maybe 8 wheels. So there'd be more resistance just because of the number of wheels, even if the LRT car weighed the same as the bus.
For air resistance, it's mostly a matter of frontal cross section and then "skin effect" along the surfaces that the air blows past. Buses and trains are mostly about the same cross section, assuming a single level bus and a single level railcar. For the skin effect, a longer railcar has more "skin" on its sides than a bus.
So overall, yes, a bus on steel wheels will likely need a lot less power than an LRT car, due to being shorter and lighter. But we need not stop with my mere speculation: San Francisco and other places still have trolley buses, which are rubber tire buses powered by overhead line. So you can probably look up the power requirement for these buses and get an idea of the power system required.
And because a converted bus sits on steel wheels and would run on gentler slopes than hilly San Francisco, it would be reasonable that a lot less power than a trolley bus would be fine in your story.
Alternatively, the trains can just run slower, since that reduces the power draw as well. But that could limit your plot, since it means rural trains might be substantially limited in speed, only due to insufficient power. Which could tilt the story into one of rebuilding the electric system, which I suspect isn't what you had in mind.
This is really good to know and quite disappointing. I try to keep things grounded and at least close to reality but had no idea of the limitations here. I'll have to think on this and I might come back with questions if that's okay.
I suspect the utopian emphasis on green power, hydro, solar, and wind, will further weaken this possibility? I haven't thought much about what the grid looks like around these fringe communities (further out where the story takes place it's basically gone and homesteads and villages have to be self sufficient) but these folks could be tied to the grid or striving for self suffiency but that would probably make it even harder to provide this kind of power reliably, even if someone was making tons of the necessary hardware because a train boom is happening.
Battery electric trains can resolve a lot of the technical conflicts, since a battery averages out power demanded from the electric network. But then you need to figure out the logistics of where trains would charge (eg layover in some towns with good electricity connections) and what the battery capacities might be for your story. If we say batteries are now 10x more efficient in 100 years, then that basically solves most of the difficulties.
But we don't even need the batteries on the train. With such efficient battery technology, it would be entirely feasible to connect to a neighborhood distribution circuit, with a track-side battery that slow-charges from the grid. Then when the train whizzes by, the track-side battery dumps power onto the overhead line, which powers the train.
This latter idea is entirely feasible today but the sheer number of batteries is what makes it impractical or financially impossible. Hence why if batteries became 10x more efficient, then the idea can be implemented.
Phrased another way, to build a trackside battery in 2025 would likely take a lot of skilled engineers to build a structure about the size of one or two houses, then fill it with batteries that hopefully don't explode or catch fire. But in 100 years, the advanced battery tech for a trackside battery could conceivably fit the same capacity into the space of one of those roadside green utility boxes. And that would be a lot easier to install in a fictional future where the country is rebuilding at a grass-roots level, when large-scale coordinated engineering might not be possible.
This is an awesome explanation and I think it feels like a good way forward! I'll read up on bus power draw to make sure but I think we're approaching reasonable - especially if the towns are already setting up battery banks to manage their own power demand.
Thank you again for all your help, I really appreciate it.