Agreed, Bill. I know this is a few technological steps away from being practical for space-limited HO engines. But packing it into the big, empty 14-wheel tender of a brass Big Boy appears more than doable, especially since the module and the speaker are both in the boiler. And since I run passenger trains almost exclusively, every one with a head end car right behind the power, I'm looking forward to a few more deadrail conversions in the next couple of years.
I've often contemplated what it would take to produce "deadrail chassis" replacements for popular HO diesels. Something along the lines of a very low-mounted motor and linkage like the Overland drives have might free up enough space to make this practical. Self powered units like the Stanton drive would be best in terms of gained space, but the trade off is tractive effort, I think. Plus, right now they only come in 4-wheel configs. In 2011 at the NTS in Sacramento, when I asked NWSL about plans for a 6-wheel version, they said they were looking into it, but sadly, to date they still have none.
But, time and technology march on, and I am supremely optimistic about the future of deadrail, and think RailPro is particularly well-suited to achieving that end.
--Michael