The SIT Gilbert steamers are truly unique postwar toy trains. A few clarifications vis-à-vis what was posted above might prove useful. The SIT steam engine models were manufactured in 1946 and 47. The versions from 1946 have the full correct road name placed on the side of the tender in the prewar Gilbert fashion. The SIT Northern and 0-8-0 were built and offered in truly few numbers in 1946 ( I have held an example of each); so few that the word "rare" does apply with values to match. The 1946 No. 332 is AC. The 1947 SIT Northern (DC) is very scarce and continues to command increasingly high prices. The 1947 0-8-0 was made as either DC and AC versions which are 'tough' but much more findable and command more accessible pricing. I have an AC version in my collection. The SIT K5 and Hudson are much easier to find and are priced quite reasonably for the operator. For listings, relative scarcity, and 2012 pricing see TM's American Flyer Price and Rarity Guide, 2012 Edition.
Generally, the parts are available to get SIT steamers in operating condition. The parts for the piston configuration to replace the bellows are available and, indeed, the piston configuration is what Gilbert used and provided as a retro-fit in later 1947. To be sure, the SIT configuration is more complicated and prone to troubles than the classic SIB arrangement (1948 and later), but worth the trouble because the sight of a postwar steam engine expelling volumes of smoke while sitting in neutral is a great show for both its owner and his/her guests and puts the wimpy, wispy postwar Lionel smoke to shame.