Marx made a 34" diameter curve in addition to the sizes listed above. These sections turn up at shows pretty regularly, and usually go for very cheap. They can be identified by 5 black ties per section. These sections will let you run quite a bit more rolling stock than 27" curves and not take up very much more room. Marx also made manual and remote switches for this track, though I am told they take a bit of rework to make them work well. They were originally made so Marx's "fat wheel" single reduction motors would go through them, so they need a bit of material added to them so modern engines don't wobble going through them.
As for engines that will run on 27" curves, traditional size GG1s (at least my LTI remake) runs on them and even clears the switch machines on MPC era switches. I have a 2046 Hudson that would hit a switch machine if the switch was right after a curved section (I think; I had to alter the trackplan a bit on a layout 20 years ago) but clears all of them on my layout now. A few people commented on how nice the 2035 is, and they are nice, but I prefer the two wheel trailing truck of the 2025. I typically pull 4 2400 series passenger cars with it on my o27 loop (any more and the train overpowers my fairly small layout.) I am not much into modern era trains, but I think Lionel did good with their 0-6-0 Dockside switcher. They navigate o27 just fine, pull very well, look good and are pretty affordable.
Hope some of my rambling helps,
J White
