Menards good and bad

I bought this box car and flat car with cement truck. You can take truck off if unscrew from bottom it has several moving parts and it is very heavy. These two purchases appear very good quality for $20 and $25. I'm Very pleased with them.  I also bought 4 flatcars for $6 a piece and their quality is horrible on the couplers. I'm using O36 fastrack I don't know if that's the issue or not. But I bought them for my children to use but if they won't stay connected it doesn't matter. On the boxcar I would like to change the B in B&O to a C idk how hard that is. any pointers?