(Resistance to forward motion. Mechanical strength.) The force that is opposed to the motion of railway vehicles. It is traditionally classified in straight and horizontal resistance, and resistance due to the fortuity of the track (gradients and curves, in practice variable with the degree of performance. The horizontal resistance is in turn due to rolling (wheel-rail pair), kinematic pairs of axes and carriage, aerodynamic resistance, overpressure in the tunnel In the case of variable speed motion, an increase in resistance is also generated due to the inertia of the rotating masses
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