So, Doc and Marty hijack the locomotive at gunpoint, and then lets it crash into the Ravine. Marty/Eastwood is believed to have died in the crash.
Why isn't Doc prosecuted, convicted and jailed for this serious crime?
When he shows up in 1985 with Clara and their two sons, there is nothing indicating that he had been in prison for several years.
It was a science experiment.
shareBecause he turned it into a flying time machine.
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This was explained by Doc Brown in Part II. In the Back to the Future trilogy the theory of time travel depicted is based on parallel universes. When a change is made in the past it creates a parallel universe. Doc explains it to Marty after they return to 1985 from 2015 and Marty finds his mom married to Biff who is now living Donald Trump's life.
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The train driver was dumb. He has a masked man pointing a gun at him and asks "Is this a hold up?", so Doc tells him that a couple of outlaws dressed as him and Clint hijacked the train trying to frame them for it. They went after them, Doc managed to get off but Eastwood went over the ravine with the hijackers.
Or Doc says that him and Clint hijacked the train because some outlaws had planted explosives (presto logs) on it and they didn't have time to explain all this. Clint couldn't save the train or himself.
Take your pick. Either way, the townsfolk aren't going to think the worst of the friendly blacksmith, or the young man who just took care of Buford, and Clara, the respectable school teacher is there to back up Doc's story. And either lie would result in the ravine been named Eastwood in honour of him.