Mythbusters: Train Suction

[brakie]

Well it took a large stream of urine to get Ted the dummy knocked over...
I am not advocating testing this "myth" by whizzing on a 3rd rail but,just past on their findings..I think we all know that household 110 volts can kill you.
I think we all know to stay back from the tracks as well.
Listen..I broke a cardinal rule of mine and turn my back to the track..When I turn back around imagine my shock when I saw a NS SD unit gliding by.If I was near the tracks they would have been playing "taps" over my grave.Shoot that engine didn't even rattle the diamonds.

 

[nkp174]

I'm a PhD student in engineering, so perhaps I shall share a little.

As the train moves forward, it compresses the air in front of it...and there is a pressure drop in the air behind it. This pressure drop is the vacuum...since the air around that space has the same temperature, but a lower pressure...lower total energy, it flows into the vacuum to lower its energy. This is the same drag force as in terminal velocity (falling objects reach a maximum speed) and that slows down military shells. Hence, most modern militaries use gas assisted shells...shells that emit gas to elimate the drag produced by the shell's movement...it'll increase their range by 50%.

The reason streamlining is effective is that it reduces the compression in front of a train (or plane, or car, or ship's bow) by reducing the dead air in front of the train that is being compressed. If your tail car is also streamlined, it greatly reduces the drag by eliminating the dead zone which creates the vaccuum. You can also see this phenomenon along lakes and rivers...places where there is current out away from the bank but not in inlets...this force actually destroys dams if they aren't designed right!

If you want to feel the compression in front of a train, stand near the entrance to a subway station when a train comes out of the tunnel...you'll also feel a more powerful pull when the train finishes exiting the tunnel.

EDIT: Dave-the-Train is right about the third rail...the higher the voltage, the more resistence it can over come (also the greater distance it can jump). DC current is actually more dangerous than AC. AC cycles up and down...60 per second in the US, 50 per second in Europe...which gives you a chance to let go, but the constant DC current just fries you...and the power delivered is the same.

 

[Illus]

I've been nailed by 250 DC working on ore bridges, and it wasn't a thrilling experience. Burned a 3/4 box wrench in half, and left some nice holes in the side of my hand where it came back out. DC holds you when it gets you, AC bucks you off. Being an Electrician, I know that most electricians are killed from falls, the fact is they are usually electrocuted, and blown off whatever they are working on, and fall to the ground...