When waiting for Doc to fire the "starter-pistol" to let Marty know when to floor it for the screen of the drive-in movie theater, you hear Marty quip, "Hi-ho, Silver." What color is a DeLorean?
The lesson about the future not being written in stone also applies to Doc and his lifestyle. A man his age you wouldn't expect to make something of his remaining life and/or get married. And yet, look what he did in this movie: he became Hill Valley's best blacksmith, married Clara and had two sons. All while he's roughly 70! So the message could also count as "It's never too late to live your life".
It may be possible that Martin's death may have affected generations of McFlys to learn from history and be cautious, leading to George McFly'stimid personality in the first movie. Also, in a round-about way, Marty's recklessness may have been inherited from his great-uncle's temper.
It might seem odd that the movie would begin with a slow panning of Marty, just, sleeping. But think about it — the poor guy spent the entire previous movie running around solving problem after problem, with no breaks in between (unless you count getting knocked unconscious). With all that in mind, it's very satisfying to see him gets the rest he so very deserved!
Not to mention that, if you do the math, he'd been awake for over 24 hours straight (excluding the "almost 2 hours" courtesy of the KO from Biff's goons), with a significant amount of tiring physical activity, much of which takes place in the last three of those hours.
If you extend this further, Marty has had a hell of a couple of weeks. Week or so in the past, get back, go to sleep, wake up, see the new 1985, go to the future, go to bad 1985, go BACK to 1955, run around, go to the wild west for a week, and then back to 1985 again. Marty just can't catch a break.
Also, Marty and Jennifer don't get their weekend down at the lake. Both have spent the whole weekend time traveling and/or passed out.
This comes up in the games series as well. Episode 5 starts with Marty hardcore passed out (he still has his shoes on). When he asks why no one woke him, he's told that he looked like he really needed the rest, and asked how long it's been since he slept. He has no idea: between all the time-travel shenanigans, he has no concept of his personal time.
Marty accepts a challenge from Buford, but plans on him and Doc being out of there before Buford arrives. However, knowing Buford, if he'd arrived in Hill Valley only to find that Marty had left town, I'm pretty sure all hell would've broken loose in one form or another, affecting 1985 in a bad way. Did Marty stop to consider this? My guess is probably not, but it sure is a damn good thing Doc can't hold his whisky.
Possibly not, since after the 'gunfight' between Marty and Buford, Buford gets arrested for his actions on a previous day robbing a Pine City stagecoach. It's possible that in the original unaltered timeline, Buford committed this robbery and got away with it, possibly disappearing forever, but in the altered timeline of the movies? He turned up to Hill Valley to meet that smart-mouth Clint Eastwood punk, and even if he wasn't there, it was only about half an hour since he arrived to Marshall Strickland’s deputy turning up to arrest him for the crime he had already committed, his beef with Marty be damned. So basically, Marty being there at all may have helped a famous outlaw be brought to justice, except in one timeline it also involved manure-based humiliation
Doc takes Marty to the desert to make sure that he has not an accident while he travels back in time, citing specifically that he could hit a tree that was there in 1885. What was the first thing Marty did when he time-traveled to 1955 for the first time? He hit a tree. Fortunately, only a small one.
Why were Doc and Marty falling out? In the original movie, Marty's father George had originally been a total wimp, so Marty looked up to Doc as a father figure. But in 1955, he bonds with young George, learning they were more alike than he'd originally believed, and it leaves a lasting impact, as in the new 1985, George is a confident, successful writer. George had become Marty's father figure once again.
Crispin Glover would've also made a good casting choice: Seamus is like George: a fairly milquetoast individual who doesn't like conflict.
1955 Doc scoffed at the Japanese parts used to make the DeLorean claiming "no wonder it fell apart, the parts are from Japan!" While this was mostly a jab at the "Buy American" attitude the 80s had (when the film was made), he was justified in saying that because Japan was still recovering from World War II which ended 10 years prior. While unintentional, Doc looked worried when Marty says everything is made in Japan because it means a former enemy has become a dominant power again. If true, it's invoking the Japan Takes Over the World trope which was a major concern in the 80s (which was also referenced in the second movie).
Many people often wonder about the time-train seen in the ending scene, calling bullshit and pointing out how the Doc himself said he couldn't fix the Delorean, so how could he just build another time-machine? But I suddenly realised something: when the Doc originally tried to fix the Delorean, it couldn't be done. But he didn't have something at that point, that he did have when he gained the time-train... CLARA. When the Doc got trapped in 1885, although he found himself work and became a part of the town, he wasn't a citizen of 1885, he was still a 1985 man. Instead of just adapting to the time-period and living amongst them, the Doc was determined to have things from the future, like a building a steam-powered 'refrigerator' to make ice-tea, something he couldn't possibly have had, and using chemically enhanced wood to make his forge burn hotter and longer. Rather than abandon it, he made it happen for himself. By the time he sent the letter to Marty, he had forgone the idea of ever fixing the Delorean. Whereas Clara is a resident of 1885, with the upbringing and knowledge of the era. Doc was thinking about things in a modern way, maybe Clara was able to provide some insight from the era that enlightened him and showed him something he hadn't considered before. He kept trying to think of 1985 solutions to 1885 problems, instead of 1885 solutions. To sum up: Clara helped Doc build the time-train because Doc, as smart as he is, wasn't looking at things fourth-dimensionally.
Additionally, Doc was initially focused on repairing the DeLorean, not building an entirely new time machine. As anyone who repairs computers — or who's taken an old computer in to be repaired — can tell you, sometimes, building a machine from scratch, using an entirely new design, is more feasible than repairing a broken machine for which you can't find replacement parts. Consider that the locomotive requires an enormous flux capacitor, which would not have fit in the DeLorean. A lack of transistor technology (the parts not invented until 1947 referenced in Doc's letter), probably required Doc to scale up when designing his revised flux capacitor, requiring a much larger machine than a car to fit it in.
Funnily enough, this also makes the Time Train far more practical as a result. Doc and Marty keep running into the problem of having no fuel for the DeLorean; plutonium in the first film, and gasoline in the third. Since Doc's time train runs on steam, neither are an issue; no matter what time period they go to, they're always going to be able to find something they can chuck into the furnace and burn. Additionally, scaled-up circuits means Doc won't have to rely on someone else making his microchips; the components would now be big enough to be removed and replaced by hand.
In addition, the Time Train has a much more spacious passenger section - much better for a family of time travelers and their provisions than a cramped DeLorean.
Another consideration, Doc has a source of the most essential parts, the Flux Capacitor, in the Delorian buried nearby. Sure, it would create a time paradox for Doc to mess with it, but as long as he eventually came back and put in replacement parts to the Flux Capacitor at the least it wouldn't matter. That plus the fact that Doc has already created a means for a train to reach 88 MPH with the logs for bringing Marty back to the future, it's definitely workable. Those are the two things Doc would really need to turn a train into a time machine, and he only needs to do a jury rigged job the first time. Doc and his family could easily and probably did go to the future and work on building a more practical and robust train design that can go faster without the logs and won't start falling apart like the 1885 train did, on top of having Mr. Fusion and who knows what else.
It is also important to remember that people from the past weren't idiots. Ancient Greeks already built things like steam engines and computers, and one ancient Greek philosopher, Eratosthenes, was able to calculate the Earth's circumference before the birth of Christ. The Song Chinese had steel mills and coal mines centuries before Europe industrialized. With Doc having all his uptime knowledge and determination with a lovely and intelligent wife to help him, it isn't a stretch for him to conjure up another time machine.
On top of all this, Doc had something he never had before; the hoverboard. He had access to future tech, and he and Clara could have spent the years carefully dissecting it and reverse engineering it.
Doc and Clara falling in love over talking nerdy (and Clara's statement that she'd never met a guy like Doc before) take on a different meaning when one remembers the timeline gap. Doc comes from the eighties, where women have more freedom, and he's delighted to find a girl with his same intellectual interests. Clara comes from an era when education was less emphasized overall and where she as a woman might especially have not been expected to focus on it. She was delighted to find a man who liked that aspect of her personality.
A major plot point throughout the second and third films is Marty's trigger word "Chicken". Whenever someone calls him that (or the similar "yellow"), he's first words are "Nobody calls me chicken (yellow)". When Marty first encounters Buford in the saloon, he calls him by his nickname Mad Dog. Mad Dog is clearly a trigger word for Buford as he becomes almost psychotic at the sound of it despite Marty not meaning it in a malicious way: "Nobody calls me Mad Dog!". This could have fueled Marty's character development towards getting over his Berserk Button, as he realized he did not want to be anything like the absolute monster that was Buford.
In a meta sense, the Logo Joke at the beginning — Universal's 75th anniversary logo card, which uses past Universal logos followed by the (then) new one — fits in nicely with the tone of the film. We go right from the logo to the famous clocktower scene, followed by a return to 1955 Doc's house and lab from Part I, before switching over into an new adventure.
1985 Doc scoffs at Old Man Peabody's plan to breed pines in Hill Valley. But when Marty travels to 1885, the town is surrounded by pine forest. So Old Man Peabody's plan wasn't absurd at all, he was breeding a native crop. The fact that Hill Valley was built on a pine forest was in living memory by 1955 (moreso if Old Man Peabody was older, as his name suggests) but would be ancient history by 1985.
When Doc tries to defend himself against the Libyans in 1985 in Part I, he uses a revolver, which jammed right when he needed it. In 1885, what does he use to rescue Marty from Mad Dog Tannen? A rifle with a scope, allowing him to be a threat at a safe distance. Plus, this helped explain how he was able to fix Clara's telescope a few scenes later.
The line between hero and villain in the franchise relates to how characters deal with conflict.
Seamus is ultimately a good guy because he knows that picking an idiotic fight isn't worth his life. He's nothing but kind and generous to a stranger.
Doc in the movie carries a rifle, but only uses it to defend Marty from Buford.
Marty, meanwhile, destroys his life because of his fear of being considered a coward, and learning to not let some asshole get under his skin is what saves his life and his future.
George is too milquetoast, causing him to be bullied well into adulthood. George learning to stand up for himself is what allows him to have a better future.
The Tannens, all three generations of them, are monsters because they pick fights to be asshole bullies.
When it comes down to it, underneath all this bluster, Buford is a pathetic weakling.
The people he's shot are the downtrodden of society who can't fight back, the "Chinamen", and a poor newspaper editor who likely didn't know how to defend himself.
Buford is willing to retreat from those who do have the ability to defend themselves or who possess authority, like Marshall Strickland and Doc, when he has shotgun ready.
Despite being armed, Buford loses relatively easily in a duel with a teenager that outsmarts him and knocks his gun away.
Buford's defeat mirrors Biff's: both went down after a few punches from someone they underestimated.
Biff's appearance at the end of the movie has him getting aggressive when he sees someone grabbing his cleaning supplies, even shouting "Butthead", until he sees the stranger is Marty. There are two ways to view this scene.
Biff hasn't reformed and is still an aggressive idiot who is only held in check by his fear of upsetting George, his employer.
Doc seems to get far more respect in 1885 than in 1985, where he's treated like a maniacal whackjob. But the 1880s would treat him far better than the 1980s: Science and intellectualism were more respected things in the late 19th century and education was not yet universal. So while an intelligent man like Doc who comes up with incredible inventions would be lost among a crowd today, he would be seen by people of 1885, especially the folk of Hill Valley, as a brilliant "man of letters". Also, the mad scientist wasn't yet a trope in 1880s fiction, so people wouldn't be inclined to see Doc as a maniacal crackpot.
So does Marty for that matter. While in most other timelines Marty's treated as an annoying teen for everyone except other teens and Doc but "Mr. Eastwood" is treated with relative respect if an oddball except for Buford who doesn't like anyone anyway. But teenage culture and adolescence was only a concept that came into effect in the 20th century. In the 1800s, people were effectively considered adults as soon as they were grown enough and sent to work, if not even sooner. The reason nobody balks at giving a 17 year old a gun or liquor or challenging in a gunfight to the death is that as far as 1885 Hill Valley is concerned, Marty is a grown man. In fact without his trendy teen fashion to give away his age which doesn't exist in 1885 they might not have any idea of his age at all beyond being an adult but not yet middle aged.
When Doc's clan travels to 1985 and finds Marty and Jennifer by the tracks, Doc's understandably surprised. He obviously didn't expect Marty to actually be right there at ground zero when they touched down. But it's more than that. Doc's arrived on the day and time of Marty's fateful drag race. Seeing an injured Marty and Jennifer here (rather than at a hospital) and Marty's undamaged Toyota would've immediately alerted Doc that history had unfolded differently. Marty had changed his own future thanks to his Character Development in 1885 — hence Doc's joyful response to the erased fax.
After Marty learned his lesson in the past, he decides not to race with Needles just because he called him "chicken", and thus prevents the accident that would ruin his life. Nice Aesop about ignoring jerks who can get you into trouble, right? Until you realize that, according to Marty's mom in 2015, Jennifer probably married Marty just because she felt sorry for him after the accident, which can mean that in this new timeline Marty Jr. and Marlene could as well never have been born...
Or Jennifer could have a higher respect for Marty and his own change. Or her knowledge of time travel and having kids in the future influencing her decision to marry him.
Take it another step forward. In the original timeline, Lorraine is really adamant against girls being the pursuers of men because "they're just asking for trouble." Lorraine of the original timeline may have been raped by Biff as well but averted by George's interference.
Fridge Brilliance: It's like how George and Lorraine met. Originally, Lorraine only fell in love with Marty's father because he was run over by a car, but in the end she falls for him anyways even though he was never hit.
Or that Marty becomes a Rock God.
Yes, Jennifer felt sorry for the Marty who constantly pitied himself after the accident, but she genuinely loves the Marty we know who doesn't race with Needles.
The Telltale game implies that they're still together at least in 2011.
Marty and Doc learn that Doc will die of a gunshot wound on September 7th 1885. They assume that he will be shot on September 7th. However, it transpires that Doc would actually have been shot on September 5th, and took two days to bleed out; it's not lingered on, but some thought about this suggests that in the original timeline, Doc's death was long, lingering, painful, and at the mercy of less-than-spectacular late nineteenth-century Old West medical practices.
Word of God in the DVD Commentary says pretty much that, omitting only the "less-than-spectacular late nineteenth-century Old West medical practices" part.
The novel also makes this point.
Doesn't Buford specifically tell Doc that the last time he shot someone with his derringer it took them two days to die?
He does, but this is foreshadowed even earlier than that, with Buford digging a gun into Doc's back, only for Doc to reply with 'you're early'. Specifically, two whole days early.
"His beloved Clara" buries him. He probably spent his dying days with her as his nurse, both of them wishing they could have had a lifetime together...
Marty has to endure 1985-A Biff: a maniac who murdered his father, abused his mother, and tried to kill him. Imagine how scared he was when the altered Biff screamed "Butthead" and got aggressive?
More-so Fridge Disappointment than horror, but because of being dragged along on all the time traveling mishaps and only coming back on the morning of October 27, Marty and Jennifer never got to go on their trip to the lake that they'd been planning for weeks and have been talking about ever since the beginning of the first movie.
