Dear Sir,
Based on your description, At least we Need 4 wire, which is 12V supply, GND, and 2 wire for RS485 UART(because RS485 is better for long wire case, and I think your case need it).
Every Trolley will have it's unique ID that can be set via DIP switch or something similar, So, you can reconfigure it. I will use arduino based microcontroller(I think Arduino Nano), max485 module, and One DC-DC buck converter module to converting 12V to 5V arduino needs.
I will deliver all your requirement(Schematics, Gerber, and 2 working code to you). Also I will provide Video of working stuff.
I have programming arduino since 2013 for various project, and personally I'm professional Electronic and firmware engineer, So this kind of stuff is my daily activity.
Schematics and PCB design can be finished in 2-3 days, Gerber BOM List in 1-2 days, usually PCB production cost 7 days(I will use Chinese manufacturer), PCB delivery cost 3 days, assembling needs 2 days, and delivering video and working code I guess needs around 3-4 days, although this is simple project, but it will always needs a troubleshooting case, So, I propose that number of days.
Lets discuss the detail over chat. Let me know how long your wire will be, and maybe the total number of trolley you have, so I can fit the DIP switch efectively.
Best Regards,
Rijal