Keywords: wear rate, subway rail, instability, wheel impact, rail fracture.

One of the most important tasks for every operator of the rail infrastructure is the optimization of cost of maintenance. With respect to rail-wear and life cycle, the quality of the rail material is crucial. Therefore, heat-treated rails with very high values in hardness increasingly replace conventional types of rails. The aim is to decrease the side- and headwear-ratio which in turn will lead to higher life cycles and lower life cycle costs based on longer maintenance intervals. In one particular section a very special situation is given: in a welded rail joint a head-hardened manganese steel rail (HSH-M) is combined with a head hardened steel rail (HSH). Such strong wear dissimilarities may introduce instability. As the wheels impact on the geometrically imperfect joint impact forces develop which force the carriage(s) to alter wheel-rail contact

Measurements of the change in the rail head geometry as a function of position and time show considerable wear at the driving edge of the rail head. As a result of the impact, the vehicle bounces off the geometrical discontinuity and the opposite wheel secondarily impacts at the inside rail of the curve, causing a marked increase in the shear force magnitude between the right hand side (rhs) wheel of the leading axle and the head surface of the inner rail. Monitoring of the evolution of vertical wear profiles at different distances beyond the corresponding weld in another series of measurements performed in March 2010 and again seven months later confirmed this statement.

A rigorous three-dimensional treatment of the impact problem of a multi-carriage train at full speed on a curved track section would require highly sophisticated dynamic finite element modeling and appropriate computing support and certainly is beyond the scope of this paper. Reducing the three-dimensional problem to a two-dimensional problem obviously means the neglect of the vertical bouncing-off effect in the impacting wheel and the associated momentary reduction and vibration of the contact force. The simplified model is based on the assumptions of the technical impact theory: the impact occurs during an infinitely short period of time, the impacting bodies are considered rigid during impact; the material behavior is taken care of by defining the impact ratio. The vehicle is modeled as a rectangular block with homogeneous mass distribution impacting obliquely on a frictional wall (simulating the left hand side rail) using the assumption that the impact occurs elasto-plastically.

As in most impact problems the quantities main interest are the total impact force generated during impact as well as the resulting post-impact movement of the impacting bodies. Obviously, the impact force increases with increasing "friction" and with increasing impact coefficient, hence, elastic impact at a badly worn-out (dissimilar) joint generates the largest impact force (just for comparison: normal impact on an infinitely rigid wall would double the impact force from plastic to elastic impact conditions. More realistic numbers for the quantities of interest would be obtained when using more sophisticated models for the impact and the carriage.

The combination of two different rail materials in a welded joint, especially in curves, should be avoided. As a result of the impact related vehicle dynamics the inner rail will also suffer excessive wear; a fact which is supported by the appearance of slip wave markings.

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