Portable derail device (100) and system for railway rails (200) configured to be releasable secured to a railway rail (200). The derail device (100) comprises a rail ramp (130) that is configured to vertically guide a wheel (300) so that the complete wheel (300) gets lifted higher up than the railway rail (200) and thereafter an "over rail" (120) guides the wheel away from the railway rail (200). The wheel (300) is guided by its normal surface of contact like when running on a normal railway rail (200). The invention is also a system for derail of rolling stock on railway rails (200). One derail device (100) is then installed on the right railway rail (200) and another one on the left railway rail (200). A pair of wheels (300) moving on the railway rails (200) will, when arriving at the two derail devices (100), first both get lifted higher than the railway rails (200) and thereafter turned away from the railway rails (200) by one of the wheels (300) getting guided by the "over rail" (120).