As for the UA Turbo, there have been a number of HO models attempted and I did buy a kit from "Novatech" in Canada some years ago, but I felt it was virtually a chunk of solid resin. It was supposed to be redone in "thin-wall" to actually have open windows (the original I had bougt instructions suggested that since the prototype was heavily tinted, just paint the windows black), but my inqueries on the "new" version never got answered and I doubt it was ever produced. Another company in Canada came along to pick it up, but again I thgink they found the original molds unsuitable. The only credible UA Turbo HO model which is still in production and is planned in later 2006 by Rapido Trains in Canada(
http://www.rapidotrains.com) which originally was to do it also with resin kit locally in Canada but from scratch (totally new mastes and molds), but last year found that it was more cost effective to produce it in China as an injection plastic ready-to-run model with varied intermediate cars as per prototype application in the USA and Canada. Having purchased some of Rapodo Trains' resin kit LRC coaches and anticipating their LRC locomotive, I have great confidence in that Rapido Trains will do good justice for the Turbo as well. So see their website for all their offerings, including a series or RTR lightweight cars for the "Super Continental", which models also are appropriate for operation in some USA roads as well. So yes, there's hope at the end of the tunnel for a good HO scale UA Turbo!
The metroliner, original and modified with pods are definitely a "why isn't Bachmann..." question. For the modified Metroliner -into-push/pull-cab-car version, I believe Impeial Hobby Productions in Philadelphia has or will have this as a resin model, as well a a number of Amtrak and commuter models.
Now as to what ever happened to..?
a) GK Models which for many years had shown in Walthers Catalogs for Amfleet cars of freelance configurations (AmRoomette, AmObservation, etc?). I think that Walthers took this line over but kept it to only the cars that were prototypical. But were the freelance Amfleet cars ever actually produced? A freeance layout for a "what-if" Amtrak service might be of interest to some modelers, does anyone have any of these, I'd be interested in them!
b) As for slot cars, most I believe were of "racing" toy games, but there is a company called "Minic Motorways" in the UK where entire street systems with cars, trucks, and buses in 00 (76:1 scale, pretty close to HO) are available, similarly like the Faller car system (true HO) is except that the Faller system gas a buried wire to follow the steering by magnetic action where Minic has slots in the road. In either case, you can animate your road or even provide a timetable bus service feeder connection to your train service.
C) When I was a school kid, my first train model was a Globe HO dummy unpainted diesel. I thought that Varney had eventuallly took over Globe.
d) Mention was made in other posts about Gilbert. It seems that Gilbert kept re-entering the HO market even recently as a year or two ago, but always seemed to fade, and get back again. I saw a website for it last time then.
e) Another frequent HO re-arrival has been Lionel. The first video camera for front onto track view in HO was Lionel's "Rail-Scope", and one of their most recent offerings was the UP Turbine, whose litigations by UP may have finally done in Lionel.
f) Another company was Cox, I have an intermodal crane, which appearance wise could be a dead ringer for an IHC crane?
There are others: Rail Chief (like the Kasiner kits, so were they in that same product line?), Samson (similar but with another material, not aluminum, which from what I've heard would crumble over time?), Blue Line, and others that I have to finish writing right now, but will try to recall which no one might have yet spoken of.
Vytautas B. Radzivanas
Perth, Western Australia