Unfortunately this is a very broad question.
Boxcars come in all different sizes from small civil war types to humongous Lionel 86' autoparts boxcars.
40' ps1welded boxcars, Lionel & MTH. The Weaver Ultraline cars seem to easily brake their corner steps.
50' MTH. Again, the Weaver Ultraline cars susceptible to breakage on the corner steps and brakewheel.
60' Atlas O Master for the details and paint.
86' lionel. Only game in town for RTR on that model. You can purchase o scale Lykens Valley wood kits.