Riding quality and safety of rail tracks are directly influenced by track geometry; hence, their degradation along time could reduce safety and cause serious accidents. Standards propose thresholds for track geometrical parameters to keep track safety and riding comfort at an acceptable level. In this study, a method is proposed to select or define a set of proper thresholds for geometrical parameter irregularities according to desirable to safety level. The impact of track geometry irregularities on the derailment index has been investigated through the finite element model. The results suggest that twist and gauge shortage have a greater effect on the derailment index compared to the vertical profile of the track. Having the critical values of geometrical irregularities that result in derailment, safety factors of Iranian and Euro code standards in determining their thresholds are calculated and compared. It is shown that each standard has a unique set of safety factors that depend on speed and geometrical parameters.