Gary Laakso asked:"Which plastic model making company offered the CMStP&P flatcar in HO scale? It likely was in the 1960s."
Ken Montero replied:
"Athearn offered it in plastic for years. Its latest iteration (out of stock) is labeled as a "heavy duty" flat car in the Roundhouse line - also as a 200 ton flat car.
I have a metal car that looks the same as the Athearn plastic car, so I suspect that Athearn also may have made the same car earlier in metal."
Jon Miller added:
"Did the Athearn car have the rivets?"
While Athearn has indeed had a heavy-duty flat car in their line since their metal model days (as well as Bachmann's six-axle knockoff), the only thing the model has in common with the Milwaukee flat in the photo is it has four trucks. The prototype for the Athearn flat is the General Steel Castings 200-ton flat car, built for several different roads including PRR and SP. Here's Tony Thompson's blog article on these cars:
Note that the GSC car is one large steel casting with no rivets nor stake pockets, which is nothing like the Milwaukee car in the original post. The Athearn model doesn't model it terribly well - the sides are too high, and the car rides too high on its trucks due to it being engineered to run on sharp curves.
Going back to Gary's question, the answer is no - nobody has done that Milwaukee car in plastic.