Are you talking about track or structures? In track design generally, AREMA provides an impact factor formula that takes into account maximum train speed and average wheel diameter. This produces a practical maximum impact load of about 1.6 times the static vertical wheel or axle load. There is an alternate dynamic load factor in computing tie-ballast bearing pressure in track having concrete crossties (sleepers). The value is based on engineering judgement, but probably should be at least 2.0.