There are two methods of accessing the interior of most Rivarossi passenger cars:
1.) There are 6 tabs on the outside edges of the underbodies (3 on each side).
... Push them inward with a small screwdriver to release them.
2.) (Easier method): The roof 'boss' is 'press-fit' into the car sides. Insert a sharp tool (e.g. X-acto blade) at the joint of the car roof & side. Slide the blade along the length of the joint while applying a twisting force. Repeat on the other side. The roof should pop off with a slight pull.
The roof and window 'glass' will separate in one piece.
The floor & seats are also a one piece affair, and can be pulled out for painting.
I had to use both methods LateStarter referenced to get the body off my Rivarossi IC set to put weights in. To make it easier on my next removal I used Renaissance wax to help prevent sticking. Have not taken them off again to see if it works but the cars do look really nice.
Rivarossi heavyweight passenger cars are very nice models, and generally excellent rollers. I have many of them that I've lettered for Southern Pacific or Pullman.
While you've got them apart, adding gray or ivory translucent window shades at random 'drawdown' heights will impart an amazing 'premium car' ambience, especially when lighted.