Lionel is apparently now selling RS5 boards. Not really worth it now as the newest ERR Railsounds boards are less money when you add in the required audio power and motherboard required for the RS5 boards. Sound is at least as good if not better than RS5 and are all generic. I looked and could not find generic RS5 boards. As for Legacy RS, the only way you will get one is buy an Legacy engine and remove its board or maybe if you are a personal friend of someone at Lionel.
Pete