I'm a retro kind of guy, so what is left of my brain is stuck in 1928. so I know steam locomotives, and am lacking of any solid knowledge about what is available in the diesel world. That said, the last known steam locmomotive kits that I am aware of were from Bowser, and from MDC before they were assimilated by Horizon Hobbies.
I can think all kind of evil thoughts about Horizon , as they discontinued all the kits, narrow gauge stuff, and spare parts, but the truth be told they were following the market, RTR stuff sells, while kits stay on the shelf now a days.
If you want to get deeply involved buy old brass and work to improve it. not cheap, or easy, and there are no instructions . check out my HON3 engine shops thread in the narrow gauge section.
I for one, am horribly disappointed, back in the late 60's as I was starting, I would look at the catalouges, and there were available drivers, frames boilers cylinder saddles, pilots cabs , tenders. I lived most of my life aqureining the skills to use all that stuff to build locomotives from piles of parts, and that stuff is no longer available. This bothers me more than the lack of flying cars. Hell it's 2012 already, we are supposed to have flying cars. that is a minor disappointment compared to the disappearance of kits, and locomotive parts for scratchbuilding. Life sometimes throws you ugly curve balls and this is one.
This is something that bothers me. I have a nice skill set at working with model mechanisms; but I acquired many of these skills building kits, and where will the next generation get those skills, I post a lot of stuff hoping they can learn some stuff from me, but I will probably just confuse them hopelesly.
Bill Nelson