At the end of Back to the Future III...

Doc Brown invents a time-travelling train to visit Marty after he travels back to 1985.

But where did this time machine get the requisite 1.21 Gigawatts of electricity, if the Mr. Fusion was attached to the DeLorean and the Doc had to work with 1885 technology?

  • 6
    For the record, the train that we see at the end of BTTF3 appears to power itself using the same fusion technology that powers the DeLorean (note also that it can fly) since it's been upgraded in the future. I suspect you're asking how Doc Brown managed to get the thing powered in the first instance.
    – Valorum
    Nov 13, 2014 at 14:49
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    Aren't there two Deloreans in the past though? One from when Doc went back and one from Marty. So whichever one Marty didn't take Doc could have pulled the Mr.Fusion from it. Nov 13, 2014 at 14:55
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    Isn't it possible the 1.21 Gigawatts was just the requirement of the particular flux capacitor he first got working in 1985, but that with time to improve the design he could also create a flux capacitor requiring less power? And another possibility is that he just used something like a lightning rod, and rode in the train at 88 mph during every thunderstorm until he got a lightning strike that supplied the needed power for him to jump to the future, where he could install a Mr. Fusion and then jump back to pick up Clara and the kids.
    – Hypnosifl
    Nov 13, 2014 at 14:58
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    @Moo - I think Matthew Green's idea is an interesting one though, Doc could temporarily take the Mr. Fusion from the DeLorean in the mine, use it to take the train to the future, buy another Mr. Fusion there to install in the train, then go back and replace the Mr. Fusion on the DeLorean just after he took it. That would seem to avoid any paradox...
    – Hypnosifl
    Nov 13, 2014 at 15:01
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    @Timmy - In the first movie he happened to have information about the exact place and time of a major lighting strike, so he could hook up a lightning rod at that place and connect it to a vehicle traveling at 88 mph at just that time. Even if he was fairly well-versed in Hill Valley history it seems unlikely he would have come across another equally specific mention of a strike and remembered it--if you looked in local histories for your area do you think you'd find anything so specific in the 1880s? If he wanted to use lightning again I think a lot of persistence and luck would be involved.
    – Hypnosifl
    Nov 13, 2014 at 16:35

10 Answers 10


In the absence of a canon confirmation (neither the film scripts, nor the cartoon seem to offer any insight) we're left with a few options:

  • The train's steam engine is generating the electricity (somehow).

The 1.21 Gigawatts doesn't need to be emitted continually but only in a single burst. It's possible that he has developed some way of generating at a lower wattage and then storing the electricity until it's needed. This is borne out by the fact that despite having been upgraded in the future, the Jules Verne Train is still puffing steam when it arrives in the present. The engine appears to be driving a spinning object housed between two magnets. If I had to guess, I'd say that this was some kind of magnetic synchronous generator:

enter image description here

  • The Doc used the Mr. Fusion from the DeLorean in the mineshaft.

Doc Brown has ready access to a working Mr. Fusion unit in storage in the copper mine. It would have been a simple matter to remove it, use it to power the makeshift flux capacitor and travel to the future, purchase a second Mr. Fusion unit and return the original to the mineshaft (hat tip to @MatthewGreen)

  • The Doc retro-engineered the Mr. Fusion from the DeLorean in the mineshaft.

Lest we forget, the Doc is a certified genius. It's not unreasonable to assume he could retro-engineer a Mr. Fusion unit (from scratch) and build one into the time-train. He seems extremely comfortable with how the unit operates and the fuel it can use.

  • Lightning strikes.

We know that you can use lightning to power the flux capacitor. It's certainly possible that he simply rode the tracks at 88mph until he got a lucky strike but the Doc would also be aware that you can vastly increase your chances of a successful hit if you fire a firework towing copper wire into a storm cloud:

enter image description here

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    Using the mineshaft Mr. Fusion would potentially be troblesome: The future into which the Doc then would have travelled were one where he wasn't saved by Marty but shot in 1885, so he'd have had only little time to obtain another Mr. Fusion and return the original one before starting to vanish
    – Zommuter
    Jul 16, 2015 at 10:59
  • I always thought that spinning device was the same perpetuum mobile I thought I'd come up with as a kid: Magnets attracting a spinning wheel but neglecting the attraction once the shovel passed by...
    – Zommuter
    Jul 16, 2015 at 11:01
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    What I'm thinking is a really big (non-flux) capacitor. Doc Brown can generate the energy a bit a a time using the steam engine and store it, to release it in one big flux-capacitor-starting zap once it reaches the required 1.21 gigawatts. Apr 12, 2016 at 18:32
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    There's another Mr. Fusion option besides the ones you mentioned. When Marty asks "Hey, Doc...where ya going now? Back to the future?" Doc replies "Nope. Already been there!" If he's already been to the future and gotten a hover-conversion on the train (as implied by the fact that it folds down its wheels in just the same manner as a flying car, and that we see it take off immediately after the line above), then quite possibly he just bought a new Mr. Fusion in the future...as to how he made the first trip from 1885 to the future, as you say he could have used a lightning rod.
    – Hypnosifl
    Apr 12, 2016 at 19:49
  • @Hypnosifl - Ah, well that's the rub isn't it. Once you have a working time machine, you've access to untold technology. Who knows what's available 1000 years later or 10,000 years later.
    – Valorum
    Apr 12, 2016 at 19:56

Did you just forget that that was a flying train without aerodynamic design, rotors and rockets?

The whole point: Doc Brown settled down in 1885 to do full scale research. He didn't need a quick solution anymore. With knowledge from future, a time machine inventor can certainly invent a 1.21 Gigawatts power source in a decade. Age of his kids should say he really did spend a long time in past.

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    I don't think he designed the flight capability in 1885, though--the last lines of the movie were Marty asking "Where are you going now? Back to the future?" and Doc replying "No. Already been there", and then immediately after we see the train's wheels fold up and it flies off. So, I think the intended implication was that he had already taken the train on a trip to the future to get it hover-converted just like he did with the DeLorean, not that he had managed to design the hover-capability himself in 1885.
    – Hypnosifl
    Nov 13, 2014 at 16:51
  • No, I didn't forget any of those things. What I remembered is that uranium was going to be incredibly hard to find in 1885, and a power generator that can produce that much energy equally impossible. Sorry, but I favor the answer above (specifically, that he could've borrowed or reverse-engineered the Mr. Fusion) over "he just had enough time".
    – Zibbobz
    Nov 13, 2014 at 16:56
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    @Zibbobz - Based on the spinning magnet thing, I'm actually favouring the first part; that the train generates its own electricity
    – Valorum
    Nov 13, 2014 at 16:57
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    To me the age of the children is key. Why would Doc procrastinate meeting again with Marty beyond needing to go to the future (once he can time-travel) and upgrade his train with flying technology (to be able to go where he pleases)?
    – Kreann
    Nov 13, 2014 at 17:33
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    "Doc Brown settled down in 1885 to do full scale research." There is merit to this. Marty received a delayed delivery letter immediately after Doc got sent to 1885. Marty discovers Doc will be killed shortly after sending the letter, goes back and alters that timeline. Doc now has more time and motivation to do all sorts of things which he may have decided to do at some point after sending the letter, except that he died.
    – Dacio
    Nov 13, 2014 at 20:13

Doc said very plainly (and very proudly), "It runs on steam!" Steam locomotives did have steam driven electrical generators on them to power the lights and other electrical appliances, both on the locomotive and on passenger cars if they happened to have electric lights, even as early as 1885.

If I remember correctly, the voltage was quite low, about 32 volts DC. However, if the Doc enlarged the generator to handle a higher load, then stepped up the voltage with transformer coils, he could get his 1.21 gigawatts purely by steam alone. You only need that amount of power for a short burst, when the Flux Capacitor activates at 88 mph, so there could be a bank of capacitors or batteries residing in the tender behind the locomotive to store up the charge.

As mentioned by Valorum, there is a spinning set of coils set between two magnets on the locomotive's left-hand side which is likely the electrical generator, and as it continues spinning even as the locomotive is at rest it is likely building up the charge to engage the flux capacitor up on the front of the locomotive as well as providing power for auxiliary functions as need be.

Digging up the DeLorean buried in 1885 for the Mr. Fusion is a non-starter for one very good reason. Suppose Doc accidentally seriously damaged it by causing a cave-in or something as he was digging it out, rendering it unusable later on in 1955 when Doc and Marty dig it out of the mine to send Marty back to 1885? That would cause a serious time paradox, so the best thing to do would be to leave it where it was (which is why in Part III Doc and Marty don't try to dig it up for spare parts after they blow out the fuel injection manifold trying to run the car's engine on the saloon's rotgut liquor).

Sitting around and waiting for a lightning strike is also a bit of a non-starter. I mean you could wait potentially for months, maybe even years for a lightning storm, and even if there was a lighting strike, it wouldn't necessarily be on the railroad tracks as you were running the time-travel train.

The easiest way is the generate the power using the locomotive's own steam to drive a generator, and if I may paraphrase Bob Zemeckis, the Doc being a man of Science, he figures out a way to make it work using the 1885 technology, and even after a visit to the future to get the train hover-converted, he chose not to mess with anything else on it, as it already worked just fine running on steam. Doc is also hinted at being a bit of a steam train fan, as when he blows the whistle on the locomotive he and Marty use to push the DeLorean up to 88mph in Part III ("I've wanted to do that all my life!"), I don't think he'd be keen to convert it to some future-centric source of power anyway, it's just more fun for it to be a steam locomotive.

  • For comparison, the amount of power we're speaking of is generated by a hydroelectric facility in modern day. I sincerely doubt that a steam train would be able to generate this ludicrous amount of power; it'd take a coal power plant to generate that amount of power. Apr 25, 2017 at 6:02

I don't think he needs needs 1.21 Gigawatts anymore. It's possible the hoverboard technology gave him clues to make time travel more energy efficient. Otherwise why would he reinvent something he concluded earlier was too dangerous? There are hints in the final dialog. Doc Brown says "The future is what you make of it..." But every time they time travel they have to fight to keep the time line from changing. Maybe the majority of that energy is polluting the time stream creating the potential for drastic changes. Reducing the time travel energy needs to only what a steam locomotive can supply may reduce the danger of changing the past or future. Otherwise I doubt Doc Brown would risk his wife and kids on time travel.


There were comics that explain this, He uses parts from the hoverboard to rebuild the time circuits and the anti gravity components to store the 1.21 jigawatt charge with a long charge time. He fitted it to this: http://backtothefuture.wikia.com/wiki/Steam_tricycle

put on a diving suit and jumped to 2015, From there he secured the parts to finish his train for its first jump were he took his family from 1899 to past 2015 again.

From there he finalised the TimeTrain and his family travelled to meet Marty in 1985.

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    Interesting stuff. Can you provide any screenshots/panels to back this up?
    – Valorum
    Jun 11, 2016 at 18:36

The line, "it runs on steam" to me does not imply that steam is generating the 1.21 gigawatts. Marty mentioned in BTTF3 that they didn't need gas because they had Mr. Fusion and Doc says that the internal combustion engine "runs" on ordinary gasoline and always has. Thus the dilemma (being out of gas) is what caused them to think of the train in the first place. It is plausible that Doc saying it runs on steam is just the method he is using to GET it up to 88 miles per hour and that he definitely has a Fusion Generator of some kind on board. No amount of steam pressure can generate that kind of power, especially with 1885 materials to hold it together.

  • The point of a capacitor is that it collects and stores energy, then releases it all at once. The Doc's steam engine can easily generate 1.21GW over time
    – Valorum
    Jan 27, 2017 at 23:04

It's simple. He used the parts he buried in 1885 to make a time machine. He just made sure to go into the future and get them to take back to 1885 so they will be there to make the time machine so he can go into the future and get the parts to take back to 1885 to make the time machine. A la Bill and Ted. Or he could have had a letter delivered to his future self like he did with Marty to have his future self drop of the parts in the past.

  • While this is possible, can you support this theory with evidence from one of the movies, or other canon sources? Otherwise, it's simply speculation.
    – RDFozz
    Jan 4, 2018 at 20:31
  • Every single "answer" here is speculation. No one really knows as there is no evidence from movies or other canon to support any of them. I believe my theory makes more sense than most.
    – Joey
    Jan 5, 2018 at 23:16
  • Yes, but most of the answers work with some sort of data from the movies or other official stories - Doc Brown has the hoverboard; there's a Mr. Fusion in the DeLorean in the mineshaft; there's an explanation in authorized comic books; etc.
    – RDFozz
    Jan 6, 2018 at 0:40

The way I've seen it is the Flux Capacitor is merely a storage/discharge component designed to safely handle the type and magnitude of energy required to make the time jump. Charging it up could be by a number of means but of course the critical function is in the discharge.

A Plutonium reactor would charge up the Flux Capacitor fairly quickly, but if you were patient (or marooned) you could take months or years to charge it up via steam.

I would assume Doc Brown's manufacturing facility would have developed to the point he could create all his own components, no doubt with Clara's assistance. I do wonder if he needed a fix 1.21 jigawatts to generate the time envelope, or if it were based on the mass of the vehicle. The train would be crammed with macrocircuits.

  • This seems like a reasonable explanation but is there any evidence you could edit in that led you to believe this?
    – TheLethalCarrot
    Nov 30, 2018 at 10:26
  • If you could take "months or years" to slowly charge up a flux capacitor without plutonium, why would the 1955 Doc be so fixated that a sudden bolt of lightning was required to send Marty back in the first film?
    – Kaz
    Jan 8, 2019 at 7:36

Has everyone forgotten the fact that he had a functional, futuristic hoverboard with him?

If we follow canon, we have to make certain assumptions:

  1. That the Delorean was no longer capable of time travel in and of itself, as it's functioning would have otherwise rendered the events of BTTF3 unnecessary. I mean sure, he liked the Wild West but he clearly wanted to inform Marty of his whereabouts and help him back 1985. Hence the letter with precise instructions which he could not, being stuck in the past, guarantee would reach him.
  2. That he was unable to efficiently replicate certain modern technologies with the 19th century parts, as his freezer that only produced 2 ice cubes took up a large portion of his workshop. An air-conditioner in a vehicle now would take up the size of a small box. If we follow this ratio, then we can assume that the electronic required for the time-machine portion of a vehicle would be insurmountably large. Furthermore, a 1985 microchip the size of a coin replicated in 1955 took up much of the hood of the Delorean. What more the 1885 version of all the circuitry needed for a separate time machine on the train?
  3. That the train runs on steam, not nuclear fusion and by extension Mr. Fusion. This one may be the hardest to accept. Doc explicitly mentions that it runs on steam and so to ponder that he chose his vehicle as a train, retrofitted it with the Mr. Fusion from the Delorean and then brought it to the future to remove the Mr. Fusion so it could run on steam like originally intended seems absurd. If we are to follow Occam's Razor, it has always run on steam and only generates it's power from steam as Doc envisioned.

What then? There was only one source of "modern/future" technology present and that was in the 2015 hoverboard. It's safe to assume that it has some sort of battery to store charge and perhaps some circuitry to aid in calibrating the rider's balance. If Doc didn't get the parts from the hoverboard itself then he could, hypothetically, have manufactured more using the hoverboard's original parts as a base. Clearly, this isn't the first solution to come to mind but there seems to be no alternative if we are to follow the logical course of events.

  • This sounds like fan-fiction/headcanon rather than an answer backed up by any sort of proof.
    – Valorum
    Oct 14, 2015 at 12:56

IMO, I don't think it was explained at all. In the first place, Doc couldn't come back in the Delorean because in 1885 they didn't have the plastic nor the technology to make the required repairs to the Delorean, else he could have just repaired the Delorean and came back in it. Since the train would have required circuitry and a flux capacitor and plastic, there is no way he could have created time travel circuitry in 1885, especially since in the letter he sent Marty he explains that the technology wouldn't be available till 1947. He obviously couldn't use the Mr Fusion from the Delorean on the train since there was a bad circuit.

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    None of the answers claim it was ever explained. The fact that he returned in the train, and that he explicitly says it runs on steam contradicts your answer! Sep 7, 2015 at 3:58
  • He also did not have enough time to continue working on the problem, as he was killed in the original timeline.
    – user001
    Sep 7, 2015 at 13:16

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