# How could the level crossing at the end of Back to the Future 3 know that the locomotive would arrive?

In the end of Back to the Future 3, Marty and Jennifer check the wreckage site of the DeLorean when the railroad level crossing suddenly activates and lowers its barriers without any trains nearby. The Doc then appears in his time locomotive machine, shares words, time-travels with his family (not to mention his weird kids, one of whom needs to learn some manners) and thus ends the utterly brilliant saga.

So the question: How could the level crossing have activated if there wasn't a train present?

(pun intended)

I am looking for an in-universe answer please as I doubt anyone would have predicted a 19th Century locomotive appearing out of nowhere at that precise time and location.

• I just always assumed it was some new tech from Doc Brown. Jul 28, 2015 at 1:38
• Yeah, Doc had been apparently been traveling on that time train for a while (since when Marty asked if he was going back to the future said he'd 'already been', apparently getting the train hover-converted), it seems plausible he wanted to make it safe for people and cars so he rigged up some device to send a signal through time to the level crossing a few moments before the time the train was set to arrive. Jul 28, 2015 at 1:55
• Doc probably had a schedule and preprogrammed multiple safety barriers ahead of time. He wouldn't have to go to each set of tracks, but he would know when he could. If he didn't show up, then somebody would try to fix the broken signal thinking it was a glitch.. Jul 28, 2015 at 3:02
• I've always thought as something like the message left to Marty at the end of the 2nd movie: a message left from the past to the train controller, forcing them to override any automatic system. Jul 28, 2015 at 15:36

Assumption - but we do see a lot of electrical activity occurring around the time machines as they shift from one time to another.

Perhaps some precursor electrical currents trigger the signal that activates the crossing.

• Agreed, this is the answer I came to put. Added related picture. Depending on how exactly the time "envelope" works, the electricity COULD precede the train by a second or two. Jul 28, 2015 at 14:10
• Most level crossings sense trains coming in a very simple way: At a section of track a little way before the level crossing, both rails are part of an incomplete circuit. When a train goes over that section of track, the axle contacts both rails, completing the circuit. Of course, Doc Brown's train didn't touch the rails due to being suspended in mid-air, but the electrical activity you note seems to include a lot of arcing, so that's probably not a problem. Oct 22, 2015 at 1:13

## It couldn't.

Electricity runs through one of the tracks. It comes into contact with the train wheels, flows through the axle, reaches the opposite wheel, and travels into the other rail. A circuit is thereby completed, and the crossing lights kick on, the bells start clanging, and the tollgate drops to block the road.

Other types of crossing signals work in slightly different ways: Some use pressure plates, a few might use induction loops, and in urban areas, some might employ electric eyes.

The one thing that all these methods have in common is that they require the train to physically be there before the warning lights come on. That wasn't the case here, so it was just movie magic.

• I'm no expert on trains but having spent a fair amount of my childhood playing on train tracks in NYC (completely unelectrocuted) I can tell you that the tracks are absolutely not electrified. There is a "third rail" that is electrified but that has a cover on it and the train has a little arm that reaches out to make contact. Jul 28, 2015 at 15:59
• @IanAuld - actually, that makes no difference. If all you want to do is check continuity (if the tracks are connected to each other through a train) then you could just measure resistance between the rails. You are using electricity to do this, but it's not high enough voltage that you'd notice if you touched it. I'm not saying this is how it works, but as an electrical engineer, I assure you that Wad's answer is reasonable. Jul 28, 2015 at 16:10
• @IanAuld - Most rails are 1/2 of a circuit, so in most cases, 1 out of every 2 rails has a mild current running through it. Just enough to be detected, not enough to kill anyone. Jul 28, 2015 at 20:25

We know that Doc has been to the future. Whether this was slightly before 2015, 2015 itself or after 2015.

My theory would be that it is probably a feature in futuristic trains that when they are approaching a level crossing, they transmit a signal to the crossing itself. As there are a lot of crossings throughout the world I would expect, it would not be reasonable to expect that all crossings would be updated to futuristic train standards, hence backwards-compatibility may have been required (i.e. a signal was sent to a crossroad that wouldn't ordinarily receive it, but this signal interfered with the operation of the device), explaining how a crossing from the 1980s would be affected by this.

Consequently, when Doc got a hold of this technology, he may have rigged the Jules Verne Time Train to emit this signal in the lead-up to its time travelling, meaning the signal appeared slightly before the train did.

Again, this is pure speculation on my part, but not an altogether implausible theory. I would welcome any comments on this theory!

• Would these futuristic trains happen to by suspended above the tracks in some way?
– SamB
Oct 31, 2015 at 21:20