All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
What you have mentioned does not affect the validity of the sale and you are not obligated to tell the buyer about it because if the railway existed during the sale, then it is something that cannot be hidden, and if it did not exist, then the occurrence of what you mentioned (i.e. the cracks in nearby houses) is just an expectation that does not count at all. Again, you have sold the land to him and he may benefit from it by building or in other purposes.
In any case, expecting an effect to occur due to the railway line that is near the land does not affect the stated sale and it is not obligatory to tell the buyer about it.
Allaah Knows best.