Here is my experience adding Legacy. My original setup included the Lionel, three way cable that allowed the TMCC Base talk to my computer and send commands to the Control Boxes that included IC Controls TPC's, ASC's and BPC's. I also had in the mix Lionel's repackaged ASC's and BPC's. Adding the Legacy Command Base required the use of the three way cable now to go the Legacy Base and cut out my computer. To alleviate that situation, I purchased the WiFi box. Since it needed the PDI type cable, I purchased the Serial Box which gave a PDI output and could now go to the WiFi Box. And since the new Legacy Boxes were connected via a PDI cable all was good. So, I think you will need to invest in the Serial Box so that you can start a PDI run and the Three way cable if you want to connect in a TMCC Base. The Serial box will also allow the running of the two wire data cable to the older boxes.
It has been a while since making the jump, but I never looked back. It is a little foggy to me but i am pretty sure that was the process and equipment I needed for the upgrade I took. Do review the Lionel Videos on You Tube. They are very good.
I should have added that I wanted to maintain my TMCC Base so I could continue to use the older Handhelds. In the beginning I used the TMCC Handheld infrequently and now I Never use them so I think the TMCC Base could be removed from my Control Board.
