Slow-speed Derailments
There are some derailments because of slow speed in tight curves, especially in freight trains with high center of gravity.
The main reason for this phenomenon is unloading in the outer wheel, which goes to a critical situation because of the larger superelevation that creates an inward acceleration, resulting in an unloading.
Because of the action of outer wheel as the steering force, this can lead to the climbing of wheel according to the Nadal formula, which expresses the relation between the lateral forces on the wheel and the vertical downforce of the wheel on the rail.
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