Do Bi-level passenger cars RK 30-6503 RK 30-6501 MTH 20-6122 fit on O-31?

When they state O31, I doubt they're considering S-curves.  As for the O22 switches, typically you'd want the switch machine on the inside of the curve if you're worried about clearance of longer rolling stock.

@Micro - According to a Price and Rarity Guide that I have for early MTH releases, both of the RailKing Amtrak product numbers to which you refer have cars which are 12.5 inches long and are capable of running on 027 curves.  As for the MTH Premier Santa Fe 60 ft. cars (20-6122), according to the Guide they are 15 inches long and are capable of running on 031 curves.

Hope this helps to put your mind at rest; notwithstanding gunrunnerjohn's comment.

@PH1975 posted:

@Micro - According to a Price and Rarity Guide that I have for early MTH releases, both of the RailKing Amtrak product numbers to which you refer have cars which are 12.5 inches long and are capable of running on 027 curves.  As for the MTH Premier Santa Fe 60 ft. cars (20-6122), according to the Guide they are 15 inches long and are capable of running on 031 curves.

Hope this helps to put your mind at rest; notwithstanding gunrunnerjohn's comment.

Thank you, I appreciate both your and GRJ' comments.
I fear that either your book or the box must be in error.  NOTE:  EBAY LINK REMOVED PER OGR Terms of Service.

I think those may be longer, I'm sure they're not 12.5" long!

Huh, interestingly I just realized that I already own a MTH/Premier 20-6122 Sleeper/Diner car which I bought way back in 1996 (but seldom run) and is part of a Santa Fe El Capitan Aluminum/red stripe set which is 60 ft. and runs on my 031 curves. 

@PH1975 posted:

Huh, interestingly I just realized that I already own a MTH/Premier 20-6122 Sleeper/Diner car which I bought way back in 1996 (but seldom run) and is part of a Santa Fe El Capitan Aluminum/red stripe set which is 60 ft. and runs on my 031 curves. 

Thank you!

The 30-6503 is now more than I'm willing to spend, and the question about the 20-6122 has been answered.

I suspect the 6501 will also get dearer than I prefer--even though the two-car set is currently 1/2 the price of the single transfer car 6503.