how do trains turn in a tight tunnel without derailing?
In: Physics
It’s the same way they turn when there is no tunnel. They just need enough clearance for the train to drift on the rail a little out of the turn for the wheel radius to be right to make the curve.
Edit: Just to clarify, for a vehicle to turn the outer wheels have to go faster than the inner wheels, because the outside circle is larger than the inside circle. Trains have an insanely elegant solution of the wheels being cones, so when the train starts to turn the inertial force pulls the train over on the tracks and the inside of the cone of the outside wheels is smaller so has to move faster to go the same distance as the inside.
It’s weird to explain a graphic of it makes it crystal clear. It’s really kind of beautiful in how simple it is.
The wheels are a form of cone which allows them to “slide” up the rail during a turn, the cone keeps the wheel on the rail.
The same way they turn on normal track. The tunnel is wide enough to accommodate the length of the train car.
If a tunnel is narrow enough, there are restrictions on how long a train car can be to go through the tunnel. Most cars do not need such restriction. Height is a much more common restriction.
Only something like a wind turbine would be long enough to have such a restriction, and they have crews constantly monitoring it throughout the journey
To clarify /u/Mammoth-Mud-9609 ‘s response, the wheels narrow on the outer side, and both wheels and the axle rotate the same speed.
So, if a train has shifted to the right on the track, the narrow part of the left wheels are on the track while the wider part of the right wheels are on the track. The left wheels will try to make the axle rotate faster, while the right wheels will try to make the axle rotate slower. Suppose the left wheels win, and the axle rotates faster, then the right wheels will move along the track faster than the left wheels, steering the train to the left and back to the center of the track. Any shift left or right will be automatically corrected.
R. P. Feynman discusses this feature in an interview: https://www.youtube.com/watch?v=y7h4OtFDnYE
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