Modified to fit an Athearn #2486 STD Observation passenger car from Dcyale's file.

When I downloaded his file, https://www.thingiverse.com/thing:5449976, it came up so-o-o-o tiny, it hardly showed up in my Cura app. So, I texted Mr. Dcyale and he suggested upping the size in Cura to 2540% which worked well except for the width. That I had to modify to 2347.83% to make it fit between the floor risers. I tried to shorten the height also but it made the interior look wrong so I ended up cutting the top of the walls (see the pics) on an angle to clear the top ridge. This cut is not seen once the car is put together.
One other thing I did was to redo the weights that this interior sits on. One of the weights overlaps the other by about 13mm (1/2"). I cut that off and flattened the bend so the weight sat flat all the way across the frame. The other cut I made was about 8mm (5/16") from the end that sticks outside the car body so it didn't extend over the steps. I than glued a piece of 2mm x 27.5mm x 7mm under the insert to make it look like a small step up from the stairs. Again, see the pics.
I did rotate the STL 45% so it fit better on my Vyper bed but if your bed is larger, put it where you want.
I hope this helps anyone that had the same size problem as I did.