I have done this awhile back, but just got another All Nation boxcar with two rail trucks and couplers. What did you do and what kind of trucks did you use to make it 3 rail?
I believe in the past, I cut off the center sill to clear truck mounted couplers. Also, think I had to cut the coupler pocket flush with the bottom of the body to allow swing.
This box is already built up, but unpainted, I am going to disassyemble to get the painted sides off then paint the roof and ends.
I have always liked the All Nation and Athearn boxcars.
Dan