Ryan: CP in 1951 beat the NP, and they had theirs built by a commercial builder. I guess slugs, by whatever name, go back further than I thought. And the certainly weren't all repurposed diesels. Thank you!
Jerry: As far as I know, the NP never made any fuss about their "engine trailers", just built them and stuck them in Pasco. I'm a little surprised that Trains would find them, unless they did an article on the Pasco yard. That was before I started buying magazines regularly, so I missed it...