For the highway ones, I don't think it's a matter of the technology necessarily being cheap, just that it's flexibility makes it worth the cost. Selling rotating ads to many advertisers (that you can program without having to replace anything physically on the sign) has got to be a better deal than selling a single ad that you have to mount and then remove when the advertiser no longer wishes to have their ad there.
I never looked into it, but an airport shuttle driver I had a few years ago started chatting with me about these things - seems he was trying to get into the business, but it's a big cost to install the things. IIRC, he claimed something on the order of 100K to purchase/install one sign (I imagine that does not include the buying or renting of the land to mount it on). But once you have it, unless there is high maintenance involved to keep it working (unknown to me, though I've certainly seen ones not working correctly several times), the cost for changing the ads is minimal.
So I doubt anything about those would translate well to model trains.
As to the Christmas store display, those could be something as simple as those digital picture frames(many can play movies as well as rotate through stills, I'm pretty sure). They have been around for quite a few years now, and the costs have likely come down. They may be something customized for displays (i.e., instead of taking a user replaceable flash card, it may be they send the video file to the display manufacturer and they pump out a couple thousand copies at a lower cost than using a consumer type frame).
While it seems wasteful, unfortunately I'm sure there will be tons of those in landfills over the coming years, since they get tossed out whether they are still functional or not.
Are digital picture frames all way too large for the purpose of a sign for our trains? I haven't looked for one specifically lately, so I don't know if any may be approaching the size of a Miller Engineering sign. Certainly there has been one in the size of a Lionel billboard, as that was one of the first Vision Line freight cars. Maybe they don't make those digital frames small enough, as most people probably want traditional hard copy picture sizes (4x6, 5x7, etc).
-Dave
