I know that most of this detail would be too small to see with normal vision in HO scale (but that certainly doesn't stop Bill from doing the incredible detailing that he does), so it makes me wonder, has anyone ever made route card decals in HO scale?   

     Once could do so, within limits. Most of them have writing too small to reasonably see, but some have a large number or initial on it, and that could be done with decals. After all, we have repack decals with truly small lettering.

      My personal approach has always been to use plain rectangles of paper, white or manila or other color. I have written a number of blog posts on the topic; if you're interested, there is a link below to a late one in the series, containing links to earlier ones.

https://modelingthesp.blogspot.com/2013/12/route-cards-8.html

The idea here, in my mind, matches Richard Hendrickson's admonition, that in some places on a model there should be SOMETHING there. A tiny white rectangle, where a route card ought to be, satisfies that for me.