Question: Doc seemed hell-bent on destroying the DeLorean. So why did he go to the future and get a hover conversion done on the train? Why didn't he just build the train, return to his own time and then destroy the train?
jshy7979
12th Aug 2021
Back to the Future Part III (1990)
Answer: He didn't return to the Old West, both of them had a desire to go to the final frontier. Their favorite author is Jules Verne, who wrote "From Earth to the Moon."
This is pure speculation, as there is nothing in the movie to support this.
Answer: Doc was happy living in the Old West but returned to the future to collect his dog, Einstein, and he didn't want Marty to worry about him. He probably also wanted to make sure that Marty had made it safely back to his own time, to properly say goodbye, and make sure the DeLorean was never used again. He never indicated he would destroy the train, only the DeLorean. The hover conversion on the train would have been done in the Old West, not in the future.
I doubt he was able to make the train hover in the old west, whilst he could easily go to the future with it and do it there, like he did with the DeLorean. He did say he has been to the future with it, so it's logical to assume that's where he upgraded it.
Doc never says he went further into the future with the train or did the hover conversion there. If he could build a time-traveling locomotive in the 1880s, then he could create a hovering one, as he had the knowledge. Marty asks if he's going back to the future, and Doc says no because he's already been there. That could be interpreted a number of ways. It's a sci-fi movie, and there is a lot of suspension of disbelief employed here.
While the movie isn't explicit about when or where the Time Train was built, other sources do indicate its hoverconversion was done in the future. While Doc could invent a machine that was capable of time travel (the mechanics of which aren't really discussed), he had to travel to the future to convert the DeLorean and couldn't even fix the DeLorean in the past.
What 'other sources' indicate Doc travelled to the future for the hover conversion? Any fan speculation is invalid. I also don't get the argument. While Doc was unable to fix the DeLorean when Marty was in the Old West, he could, and did, in later years, build the time-travel train in the past. He could not otherwise have gone anywhere into the future to do anything. Time-travelling without the hover ability would be extremely difficult as a locomotive would be noticeable and require taking off and landing on empty train tracks. Doc would have to hide the locomotive while converting it. He would also have to know before time-travelling that the railroad tracks he took off on still existed in the future, as he could possibly arrive smashing into what became an urban development. This should be considered as both a deliberate plot hole and a plot device using "suspension of disbelief" solely intended to give the series a spectacular finale.
The comics reveal that Doc Brown traveled to 2017 in a prototype time machine and purchased materials which he brought back with him to the 1890s to use on the Time Train.
26th Mar 2018
Back to the Future Part III (1990)
Question: Doc is a scientist right? The DeLorean had a ruptured fuel line and needed gas which, as Doc pointed out, was unavailable at the time. Surely he knew how to distill booze to make ethanol? (There was plenty of whiskey around at the time). I mean, they've used it to power cars in Mexico for ages. Why didn't Doc suggest this?
Answer: They did try it. Doc ran the engine with the strongest thing the bartender could find them and it blew out the engine. It takes all the power a heavy car like a DeLorean has to get up to 88 MPH. It wouldn't be able to get that much power running on ethanol, in addition to the damage caused to the engine.
Answer: Putting to the side for a second the possibility that he either did not know how to do this, or simply didn't think of it... a quick Google search says that the entire process to produce ethanol would take about a week. They had about 3 days because they were trying to leave before Doc got killed.
3rd Aug 2010
Back to the Future Part III (1990)
Question: When Marty and Doc are on the train at the end, and Clara shows up, Doc says that Clara will have to go with them to 1985. Why does Doc say that? Someone submitted a correction saying that Clara is better off in 1885 because she was supposed to die so staying in her own period is better than going to the future, so why would Doc suggest such a thing?
Answer: Simply because, believe it or not, it's hard for Doc to kill someone through inaction. He saved her life when she was supposed to die. But that doesn't mean it'd be easy for him to do nothing now and just let her die when he knows he can do something to save her. And by taking her to the future, he is likely thinking he can avoid any other complications that may arise from the fact that she is still alive when she's already supposed to have died.
Answer: They literally had no choice but to take her with them! They had gone past the windmill so they didn't have enough track left to stop the train before it went over the ravine so that's not an option. Doc and Marty are not murderers, they are not just gonna leave her on the train so that she dies! Plus, let's say they didn't care what happens to Clara, you've still got a problem... Clara is in the cab, she has the controls! How long before she just starts pulling random levers, turning random valves etc whilst trying to work out how to stop the train? If the train slows down at all, they will not have enough time to get it back up to speed... Them, the train and the DeLorean would be at the bottom of the ravine.
12th Mar 2020
Back to the Future Part III (1990)
Question: Doc is quite a resourceful and clever guy. Why didn't he set to work on repairing the flying circuits which would have enabled them to use Mr Fusion to reach 88mph, instead of the engine?
Answer: Mr fusion only powers the flux capacitor. The engine is needed to get the car up to 88mph whether flying or not and the only way to get the car any power is by the use of petrol, which didn't exist in 1885.
At the beginning of the movie, when 1955 Doc reads the letter that 1985 Doc sent to Marty, he reads that the lightning bolt activated the time circuits and at the same time destroyed the flying circuits. Because of this, the Delorean will never fly again.
These answers are correct. Plus, to the original question: as clever as Doc is, keep in mind he got the flying conversion done in 2015. Definitely no way he would have been able to repair something so futuristic with 1885 tools at his disposal. He couldn't even get gas.
Yet just a few years later he had built from scratch a flying time-traveling locomotive, all with 1885 tools and parts.
There's no indication he built the flying train in 1885. It's suggested he had been time traveling with his wife and kids and says he's already been to the future. Whether this is in the DeLorean or the train it's not clear, but the dialogue suggests he's been to the future in his train with the family and could have modified his train to fly with future technology.
19th Aug 2019
Back to the Future Part III (1990)
Question: Back in 1885 why doesn't Doc change the letter he sent to Marty, asking him to bring a can of gas?
Answer: When Marty received the letter from Doc in 1955, as seen in the second movie, Doc wrote down that he didn't want Marty to go to 1885 to rescue him because he was happy living in the past. Instead, he wanted Marty to take the Delorean straight back to 1985 and then destroy it so it could never be used for personal gain again.
But once Marty appears in the past Doc could easily change the letter, changing things such that Marty would bring gas with him.
Answer: This would create a different timeline, not the timeline they are in.
Answer: That would not be possible as in 1885, Doc sent the letter on September 1st, and 1955 Doc sent Marty to 1885 on September 2nd so it was a day later and on the 1st, Doc was not expecting Marty to turn up. However, one CAN ask why Marty and Doc didn't go to the local Western Union office and change it (or write a new one) there since it was in their possession per the gentleman in part 2.
Changing the letter while Marty is in 1885 with Doc would accomplish nothing, because it doesn't it instantly travel to the future. Marty at the end of Part II, for his part, may receive the letter almost immediately, but the letter itself had to wait 70 years to be delivered to him.
I mean, there's no solid rules to time traveling, but just for argument's sake it seems like the letter idea could work... in the franchise, when something is set in motion, the effects usually take place immediately. Take for instance when George and Lorraine kissed at the dance in Part 1. The picture of Marty and his siblings went right back to normal, even though the kids had not been born yet. Doc and Marty changing the Western Union letter "could" have had an immediate effect and a gas can could have materialized in the Delorean, much like we've seen newspaper headlines change before our very eyes, disappearing gravestones, etc.
In your examples, the changes occur to future events. The items that changes, like the picture and newspaper, are from the future themselves. They can't change the past by changing events in the future (like they do in Bill and Ted's). This is why Doc and Marty couldn't go back to 2015 to stop old Biff from taking the DeLorean.
Answer: Quite a bit of time has passed for Doc since Marty went back to the future; he and Clara are married and have two children who look between six and ten years old. Plenty of time for him to change his mind and decide he likes the time traveling life with his family.