I've been playing around with the HC-12 wireless serial modules. These are tiny modules provide a very simple wireless serial link; all you need to do is connect power (3.2- 5V), ground, and a TTL serial data line. They cost between $4 - $5 each and have a range up to 1 km in open air:
It occurred to me that you could use them to transmit TMCC/Legacy commands to any number of receivers in accessories, rolling stock, or even locomotives. So I tried it out and it worked: I had one HC-12 module receiving serial commands from the Legacy base serial interface (converted from RS-232 to TTL), and other HC-12 modules receiving the TMCC commands over the air and sending them on to an Arduino or a laptop serial monitor.
What I'm envisioning is you combine the HC-12 with an Arduino Pro Mini (~$2-$5) and a 5V supply (~$1) and you have a wireless receiver that process TMCC or Legacy commands and take any action you can envision an Arduino taking.. Sort of like an ERR Mini Commander, but a quarter of the price and infinitely programmable (albeit lacking built-in triacs).
You could use the combination to control a very complex accessory or operating rolling stock from the CAB-2... my problem is that I'm not creative enough to think of an accessory or operating car that I would want to automate. So right now it's a solution in search of an application!