FALL IN FRONT OF TRAIN: THE PISTON EFFECT

When a train passes through a tunnel, it pushes air ahead of it out the tunnel, acting like a piston. The air emerges like water out of a hose, almost in a straight line path along the tracks. If a person stands on the edge of the platform then the high speed blast or air, at low pressure, can “suck” the person into the path of the oncoming train.

A film showing the piston effect can be seen here. A 50mm diameter aluminum cylinder was dropped down a 60 cm long plastic tube. Two strips of paper were mounted in the path of the cylinder, to indicate the air flow out of the tube. The result was filmed at 300 fps and saved as an H.264 compressed QuickTime movie (1.8Mb). If the paper strips are moved horizontally so the tip is beyond the edge of the tube, the air flow is much weaker.

The effect on a passenger was simulated by a vertical strip of paper located near the bottom of the tube. The force on the paper is smaller than before, but it shows clearly that it is dangerous to stand near the edge of the platform, especially where the train emerges from the tunnel. There is a small outward force when air starts to emerge from the bottom end, followed by a larger inward force due to the increasing speed of the air emerging from the tube (the “train” accelerates as it falls through the tube). Bike riders experience a similar sideways force when a truck passes nearby. If the air speed is increased 10 times, the change in air pressure will be 100 times greater. If the area of the paper strip is also increased 400 times to equal the area of a person, then the sideways force will increase by a factor of 40,000.

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