On small lines with not a lot of spending money, they stuck in painted timber mileposts at the outset to satify the Board of Trade inspection until taken over by a larger company which replaced them with something more permanent, usually in the larger company's standard design.
On closure, fixtures and fittings would be removed for reuse elsewhere, put up for auction or sold for scrap. You'd still find large stone GSWR and CBSCR and concrete DWWR mileposts along trackbeds as they were more trouble than they were worth to remove.