Bryan: The Midland would definitely have had to upgrade its rail. The D&RGW had to lay heaver rail when they got their K-36s, and I suspect the early Midland track wasn't much heavier, in spite of being standard gauge.
On the other hand, code 40 is plenty tall enough for modern rail, so you could run an SD without it looking out of place. It might be interesting to have a couple, just in case you find a few modern cars that look interesting, and want to be able to run them.
