The answers and conceptions used on this page may NOT be official. They are discussed and used on the forum at BTTF.com. All answers are modified to be suitable for this FAQ, but the idea are credited with the username that were provided at the moment it was collected from BTTF forum.
5.1: How come they did not need 1.21 Gigawatts in BTTF3 to timetravel when they push the time machine by train?
Mr.Fusion on the DeLoreon was still working in 1885. Mr. Fusion is a mini-nuclear-generator ('fusion' is a type of nuclear reaction), that's why the time machine no longer needed plutonium rods. As Doc said when he found out Marty had tore a hole in the gas tank:
”Mr Fusion powers the time circuits and the flux capacitor. But the internal combustion engine runs on ordinary gasoline, it always has. There's not going to be a gas station around here until sometime in the next century. Without gasoline, we can't get the DeLorean up to 88 miles per hour.” (QXR37 – www.bttf.com and bttf movie)
5.2: When Doc and Marty left Einstein in A1985 when going back to 1955. What happened to him? How did he turn up with Doc in his train at the end of BTTF3?
Logically the Alternate 1985 we see in the movie ceased to exist at the end of BTTF2. Some may argue that since there is only "one" timeline at a time, the universe change and living organism are sort of evolved when the time is altered. When Doc and Marty are going back to 1955, and further back to 1885 it would create several new timelines. Since there are two Einstein's in A1985 it means that one of them have to disappear. Since Einstein from the original 1985 are the "timetraveler" he sort of have precedence over Einstein from A1985. Somehow, after Doc traveled from 1885 he had to return to 1985, maybe even on the exact time and place and pick up Einstein before he met Marty at the end of BTTF3.
5.3: When 2015 becomes present, will Doc and Marty show up as they do in BTTF2?
There are two possibilities:
- Alternative 1: We can assume that he does turn up so space and time continuum becomes balanced. In the present we see him traveling into the future, so eventually he will show up. (mr.xl)
- Alternative 2: Like Doc Brown states, the future is not written yet, it's whatever you make out of it in the past. When they travel to the future it's only creates a most likely future, so we can assume that we know nothing of the future from the point after BTTF3. (mr.xl)