My suggestion would be that you put the B&O cars up for sale, sooner rather than later.
The definative set of scale B&O Columbian cars will be the Golden Gate Depot set when the B set issues at the end of the year (set A - the coaches have already shipped.) The GGD set are expensive (retailing for about $200 a car), but my guess is that when the B set comes out (observation, dome, diner, combine), that some folks who currently hold the K-Line 21" cars will put them up for sale.
Jim