In the US, it was only required in 1952 (due to the aforementioned law passed in 1947), and there was very little experience with it before then. By 1947, it was clear that diesel was going to replace steam due to steam's significantly higher maintenance costs (and the lack of MU). If you're running a mainline railroad in 1947, you already know that you're gonna be replacing almost all your steam locomotives within the next 10 years, so why bother upgrading them?