Many boxcar models, whether kits or ready-to-run items, allow the doors to be opened but then do not have a detailed interior. Not to worry - it is not that difficult to add one. I was building a Chivers kit which included a detailed car floor but the interior walls were blank.
I added an interior to the car during construction as show in the steps below. The car I was modelling was freelance so I simply chose a layout that looked right to me, if you are modelling a specific car you may want to do some research, but the basic method should still be appropriate.
This shows the car partly built as per the kit instructions. The lack of detailing on the interior walls can be seen. As I was going to be modelling the car with only one door open I would only add the interior to the ends plus the side that could be seen.
Looking at the car from the other side this view shows the first stages of adding the interior. As mentioned above this is a generic interior rather than a specific model of an actual car.
The door, sides and ends have been lined with scribed Slaters plasticard chosen to have a similar plank width to the exterior. The planking backing the door is cut separately so that when looking in the car it still appears that the door would be able to slide past the sides.
The framing is Slaters 0.040 x 0.080 microstip (I actually used two layers of 0.020 x 0.080 as that is all I had at the time).
In this view the inner planking has been added. For this I used a wider planking than for that representing exterior, again do what you think looks right or check prototype plans. The top of the horizontal planking should really coincide with horizontal framing, but as it will be almost impossible to see this when the car roof is fitted I chose not to add it.
This view shows the interior after painting in wood coloured enamels
Finally the interior has been weathered. As it will only be visible through an open door this should do the trick, but do look out for a future section on how to make a far better job of representing weathered unpainted wood.
For more images of the boxcar, still under construction, check the Riverside Yard Rolling Stock page.