Carey:
These special issues like this are all new material. In effect, it's a collection of the "best" of the new submissions to MR that all deal with a certain subject. In some cases, I believe they even solicit their regular authors for content on a given subject.
In my case, Andy Sperandeo saw some photos on my web site and contacted me, asking if I could do an article on my ballasting and track weathering techniques, which I did. And to my surprise, they felt the material was good enough that it ought to be the cover story of their special issue.
As part of the submission, I threw in a professionally produced 5 minute video demoing the techniques I discuss in my article. MR loved the video and has asked me for more. We're currently discussing how this would all work since video content is a new area for them. In the case of this special issue, if you get the issue, you get an access code to download and watch the video for free as a bonus.
I'm expecting to see MR branch out into more multimedia products on the hobby over time, and since I am producing how-to videos for the hobby myself professionally, MR's talking with me about what else I could do for them.