well for prelimns in the rrb exam includes the nationality of india or neighbouring countries including nepal,bhutan or a tibetan refugee.
along with that based on scale of the post applied for. let us say for scale 1 it should be more than 18 years old, for scale 2 and scalle 3 they need more than 21 years of age. and later on for scale 3 and scale 2 you need to be a graduate but for scale 1 you just need to have an intermediate level education.