The correct answer is: A. Longitudinal movement of rail.
Creep is the slow, permanent deformation of a rail under the action of repeated loading. It is caused by the combined effects of the rail’s own weight, the weight of the train, and the friction between the rail and the wheel. Creep can cause the rail to become wavy, and can also lead to the formation of cracks.
Option B, lateral movement of rail, is called gauge widening. It is caused by the same forces that cause creep, but it is a different phenomenon. Gauge widening is the lateral movement of the rails relative to each other, while creep is the longitudinal movement of the rails.
Option C, vertical movement of rail, is called rail buckling. It is caused by the same forces that cause creep, but it is a different phenomenon. Rail buckling is the vertical movement of the rails relative to the ground, while creep is the longitudinal movement of the rails.
Option D, difference in level of two rails, is called rail misalignment. It is caused by the same forces that cause creep, but it is a different phenomenon. Rail misalignment is the difference in the elevation of the two rails, while creep is the longitudinal movement of the rails.