I have several sets of Weaver and MTH passenger cars that I want to upgrade with interior passengers, seating and LED lighting. The original Weaver Wabash passenger car is shown here.
I used a CAD program to design a set of seats for the passenger car and 3D printed them.
The seats were glued to the frame and eBay seated passengers were added. I had to amputate feet to get the passengers to fit.
There is a channel running under the seats for wiring both pickups. The strip of LEDs were powered by design based on a Gunrunnerjohn's post. I designed the PCB board and fabbed it at PCBWay and I used components purchased on eBay.
The result looks better than the picture. Not counting my time I can upgrade a passenger car for about $5.