Why did the Doc decide to use a train?

In Back to the Future III, Marty gets back to 1985 by putting the DeLorean in front of a train, then using special fuel to increase the speed of the train. If this failed, the DeLorean - with Marty inside - would fly off a cliff, killing him.

Why did the Doc choose this method, which was incredibly dangerous?

If the goal is simply to reach 88 miles per hour, why not have horses pull a rope through a rope and pulley system, with several pulleys, which would exponentially increase the speed provided by the horses? Or since they're in the Sierra Nevada Mountains, why not let the DeLorean roll down a steep slope? Almost anything would be safer than using a train with explosive fuel to push the car towards a huge cliff.

• I haven't done the math, so I couldn't swear to it, but I doubt a horse-and-pulley system would work; the materials wouldn't be strong enough. And I don't think you'd find a slope that was simultaneously steep enough, flat enough, and long enough to maintain control of the vehicle. Commented Oct 22, 2015 at 1:47
• A pulley system can convert a high pull rate at low force into a low pull rate at high force. They cannot be rigged the other way around. For that, you'd need something like a geared system. I'm pretty sure the Doc. explicitly mentioned (and rejected) a hill. I always wondered why he did not solder a couple of nozzles onto the end of a tube of gun powder and strap those onto the Delorean (then run it along the rail line). Commented Oct 22, 2015 at 2:04
• @AndrewThompson - I didn't know that, and I don't understand it, but I believe you. Commented Oct 22, 2015 at 2:28
• @AndrewThompson, that's not true. Say you had a block and tackle system, you could pull on the rope to get a slow but high force on the tackle, or you could pull on the tackle to get a fast but weak pull on the rope. Commented Oct 22, 2015 at 4:45
• @MichaelItzoe, A) terminal velocity for a human body is around 120 MPH (Wikipedia). I'm going to WAG that a DeLorean has a higher mass-to-surface ratio, and is more aerodynamic, so it's terminal velocity would be higher... in any event, I can't imagine it being less than 88 mph. B) It could also depend on whether the DeLorean was manufactured for the African or European market. Commented Oct 23, 2015 at 11:33

Because train tracks were the only smooth, regular surface available.

If you've ever driven down a dirt road, you know that they're really not great for high speed. And as a sports car, the DeLorean would have a low undercarriage and stiff suspension - it would have either bounced off the road or grounded out very quickly.

Train tracks, on the other hand, were laid to be smooth and even, more so than anything else in that time and place.

Trying to create a smooth enough road would have been time- and labor-prohibitive.

Trying to build a non-train propulsion system for the DeLorean that would use the tracks (as @AndrewThompson commented above) would have been time-prohibitive and, quite frankly, hella more dangerous (if you want to build a rocket propulsion system, plan on having some spectacular test failures early on in the process!)

Simply put, the train tracks were the right surface and the train was the best propulsion available.

• Another reason to use the tracks (versus rolling down a hill or using a pulley system) is that Doc and Marty knew that the tracks were still there in 1985. They didn't need to worry about traveling through time and coming out in someone's house or worse. Commented Mar 14, 2016 at 22:55

Possibly to avoid 'temporal contamination'

True, using a train to push the DeLorean to 88mph is risky, but consider having it go down a hill. There is the real potential that pushing it down a hill or mountain simply wouldn't get it up to 88mph in time. If it didn't and they crashed this could have disastrous consequences. Imagine the wreck of a DeLorean (not to mention a time machine) was found at the base of a mountain. If it were discovered, it could have a severe impact, not only on culture (aliens are coming!!), but also technologically (a car from 100 years into the future - pretty impressive).

Regarding the horse-and-pulley system, again this has huge potential for temporal contamination. If something goes wrong and the DeLorean isn't suspended properly, it could shoot out into plain site of a bunch of people. Again, the civilisation of Hill Valley will be temporally contaminated.

Also, remember that:

this sucker's nuclear

so, if it exploded and wasn't far away from the Hill Valley settlement, it could destroy Hill Valley. Then how will Doc and Marty meet, let alone embark on their adventure, if Hill Valley was destroyed by a nuclear explosion?

...but by using the train

Yes, the issue of temporal contamination still remains by using a train, but they knew it was possible to get the train up to the speed. Furthermore, if something went wrong and the DeLorean didn't get to 88mph, the DeLorean would go flying over the edge, meaning it would be very difficult to find the remains of it, reducing the chances for temporal contamination!

• Any comment as to why this was downvoted? Please let me know of any flaws in this post and I'd be happy to talk about them! Commented Oct 22, 2015 at 1:57
• The DeLorean would be moving if they used pulleys. The ropes and pulleys would be dragging it forwards. Commented Oct 22, 2015 at 2:12
• @WadCheber ah I see - misunderstood that! Commented Oct 22, 2015 at 2:25
• 1) If the train plan failed, there would still be pieces of DeLorean (and the flux capacitor, Marty, etc.) in the canyon. Is that really so much better than a wrecked DeLorean at the bottom of a hill? 2) In BTTF3 the time circuits run on Mr Fusion, which presumably isn't all that prone to causing gigantic explosions. 3) Out-of-universe, this is a textbook example of Rule of Cool. Commented Oct 22, 2015 at 8:33
• @RoyalCanadianBandit in response to 1) yes it is much better - it's much easier to find wreckage if it's at the bottom of a mountain/hill than in a canyon. I'm pretty sure there would be less people walking around a canyon than there would be at the base of a mountain Commented Oct 22, 2015 at 22:35

This is pretty much spelled out in the film. The train was the only thing that could get up to 88 MPH in 1885, but they needed a stretch of track long enough and straight enough to accelerate the train to that speed without derailing it. Unfortunately, the only suitable stretch of track was incomplete. They could get the train up to 88 MPH in time, but the margin of error was slimmer than they had hoped.

The big question is why go through with it? Biff was going to jail, and the threat seemed to have been resolved. They could have at least waited for the bridge to be completed or traveled to another area with straight track.

• To your second question, the most likely reason they rushed to still complete the journey that morning was because they didn't want to risk altering the timeline any more than they already had. Commented Mar 14, 2016 at 22:57