Actually, if I'm not mistaken the RTS is (was?) available from Pirate Models in metal-kit form in both HO and N scales.
Take a closer look at M630's post. The point he was making is that the Pirate models, being in the $65-70 range, are not really a viable choice for someone wanting to add a fleet of RTS buses to their layout. Additionally, while they do have two versions of the RTS available (so-called Mark III and Mark IV, if I remember correctly), there are actually many different configurations of these buses in service around the country.
For example, while I believe all of the Boston MBTA's 800-odd RTS buses are part of the Mark III line, there are quite a few detail variations among the fleet. Paul Revere Transportation, a private operator also in Boston, uses nearly-identical buses that have a different style of doors. I'm not suggesting that there should be a series of kits allowing me to replicate every detail difference between each group of buses, since after all we primarily model trains, but the point is that the Pirate model would require a lot of re-working, if I could even afford one. It would just be much nicer to have a multi-piece plastic or resin kit that could be custom-assembled following a certain prototype.
There is actually a German company called
MEK that has an extensive line of HO scale buses, including the MC-5C, MC-8 and MC-9. One each of the latter two was up on eBay last week, and they appear to be highly-detailed resin kits that include Greyhound decals. They're listed at 34 Euros on MEK's site, provided you can find your way to the price list (Preisliste), but I've heard that it's difficult to order those kits from here in the US. From the looks of it they'd be the most likely company to eventually offer more modern American city buses. Unfortunately, these still aren't the mass-produced plastic models we'd love to see.