Background, I recently was modifying an engine with various parts I was able to source. I had on hand 3 different LCRU 2 boards and an ERR Sound Commander 1.5 and all the cabling. What I ran into was the dreaded works in conventional, but the sound commander goes into conventional mode when the system is working from TMCC. I saw previous threads on the serial data buffer using an emitter follower circuit, but decided there must be a way since all 3 LCRU boards did the same thing. Then I found the secret- R33, Lionel used a 10K Ohm resistor between the processor and the output data pin connector. 10K is extremely safe to ensure not blowing the processor pin, but at the same time, very limiting drive wise for any serial data device attached that basically must have some sort of low impedance buffer. So all I did was swap that 10K surface mount resistor for a 1K (borrowed from a broken and parts robbed lionchief remote) so at least I'm using genuine Lionel parts when I mod.
The factory (R33) resistor was labeled 103 (10K), the replacement is 102 (1k)