Logs are 3 7/8 inches long
Two diameters: 3/4 inch and 9/16 inch

Just my view and experience -- opinions will vary.  First suggestion is you join Yahoo On30 Conspiracy forum where a lot of good info can be found.
Specific questions:
   Bachmann 2-8-0 will fit on a 12 inch turntable.
   Freight cars are 7 inches long
   Passenger cars are 9 inches
   Centerline track spacing that I use is 2.5 inches straight, 3 inches curves.
   I use a minimum radius of 26 inches -- all Bachmann engines will operate on 18 inch radius, but the Forney especially has problems because of excessive rear overhang.  I have several Forneys (2-4-4T) so I go for 26 inch radius and larger when possible.
   Just some factoids to get you started.
        Alan Miller, Bainbridge Island WA

Hi New Guy:
   It is really very simple.  On30 is just O scale (1/4" to the foot) but narrow gauge.  The gauge is adjusted so that it runs on HO scale standard guage track.  Everything else is O scale.
   AlanM

Dave:
   I can get things started:  "low rider" trucks, as on the log cars have 22" wheels.  Standard freight trucks have 24" as do the passenger car trucks.  Tender trucks on the consolidation seem to be 26".
    Alan

B'mann 2-8-0: I make it 1 - 3/16 inches diameter at the top of the running boards.

I make it 9 5/8 inches knuckle face to knuckle face.
   Alan M

Reminds me of my favorite story of the SS&RL, 2 foot gauge line in Maine.  Engineer got her up to 60 mph and the conductory made his way forward and suggested he slow down as all the passengers were on their knees in the isles praying.  Could be done, but maybe not a good idea.
   Alan Miller, Bainbridge Island, WA

Pullman Green by Floquil is a good match for the unlettered Bachmann coaches I have found.  But the C & S color is a bit darker it looks to me and I have not tried to match it.
  Alan Miller
   Bainbridge Island, WA

Since the original thread on this subject (labeled "new poster") has drifted off this subject, I would like to get back to it with a new and more informative title.  I have enjoyed the postings that addressed the physics of whether or not a steam locomotive would be more efficient, less efficient, on unaffected by high altitude.  However, it seems to me that there must be a good deal of folk wisdom on the subject.  Firemen and engineers who actually ran the locomotives must have had some opinions, whether or not they might be scientifically correct.  Anybody know where such lore might be found?
     Alan Miller
     Bainbridge Island, WA

Steam efficiency at high altitudes is a question that I have wondered about too.  Physics is not my strong suit, but to start the conversation I would speculate that the amount of work you can get out of an engine is a function of the heat energy it contains.  Since water boils at a lower temperature at higher altitudes, it would contain less heat energy.  So wouldn't it be less efficient?  Anybody out there that understands thermodynamics?

One simple thing you can do is to check the underside of the engine to make sure there is no packing material fastened or lodged there. 

Royce:
You and I are of one mind regarding painted ladies.  But have been looking for some time for flanged domes and can't find a supplier for On30/On3.  Do you have a secret source?
    Alan Miller

Thanks, Tim.  Just what I wanted to know.  Is the D-valve the same as a slide valve, perhaps?  I have heard that term.
     Alan

I have a question about how to talk about types of cylinders on a steam engine.  I am doing a kitbash that needs to replace the more modern type with the older one.  My problem: what do I call the "2 tootsie rolls tied together with sheet metal" modern type vs. the "single tootsie roll with a square box on top type"?  It is embarrassing to talk like that.  These things must have names.
     Alan

As a kid growing up in central Kansas in late 40's and early 50's, I had a lot to do with these grain doors.  Our elevator had an inclined track just past it and the full cars would be allowed to drift down to join the larger bunch; they even let me ride the cars.  Perhaps trying to get rid of me?
   In any case the boards would be stacked outside near where they would be used, rather like stacks of railroad ties, except more loosely, with each course at right angles to the last and leaving holes between, a detail that would be good to model.  I remember a young friend once, having got into his dad's whisky supply and getting very drunk and very sick, using one of the holes as a privy.  Still don't know if they used the boards afterwards.  Enjoy your bread.
  Alan Miller