Thomas,
I recently purchased RS 5.0 boards from Lionel to retrofit a couple of TMCC steamers that I upgraded to ERR cruise M. These engines had earlier RS 2.0 boards. The 5.0 boards I put in the steamers made a huge difference.
I don't believe you can purchase the Legacy boards at this time, however the 4.0 & 5.0 boards can be purchased through regular customer service. Just have a part # available.
BT