tap a scraper down the edge of the dado rail if its nailed then it should come away easily, but its rare to just nail on a dado rail without glue.....start at an end ( on an external corner or up against the architrave of the door ) tapping the scraper as far into the back of the dado as you can then prize away STEADILY......move along the dado a little at a time, prize too hard and you could pull away the plaster, try not to use a screwdriver, chisel or similar as leverage or you will just dig into the wall.....there will be an amount of filler work once the rail is off but patience in the removal proses will minimise this