This is an interesting discussion, but I'm not clear on what role the Arduino is playing (at the risk of asking a stupid question...) - where does it "sit" in the operator - locomotive communications scheme? In an 'ordinary' TMCC scheme, the operator has a handheld controller, which communicates 'wirelessly' with the TMCC controller, which then communicates with the loco. Does the Arduino talk with TMCC controller or does it eliminate the TMCC controller by sitting in the loco/tender and talk directly to the TMCC board in the loco/tender? In the latter case, then presumably one can talk with the Arduino via bluetooth and a phone app?
The straightforward role is to have a single Arduino talk to the Command Base via the serial port, and let the Command Base communicate with locomotives, switches, accessories, etc. I have used that approach to create a complex signaling and automated train control system that you can find here.
One could theoretically install small Arduinos into locomotives, but if you're starting with command control locomotives I'm not sure what the advantage would be over having your Arduino use the existing TMCC communication infrastructure.
I did create an Arduino-based board to install in operating cars (sort of a replacement for the ERR Mini Commander), but used a master Arduino node connected to the Command Base to read TMCC commands and transmit them over a radio link to nodes installed in the operating cars (documented here).