Malcolm,
I have maybe 36 freight cars and 6 engines. I'm expanding my 11x12 layout to include a 3-track "fiddle" yard so I can have a spot to build a train. At present the yard will only hold 16 cars and the sidings I have can hold maybe 20. I may be able to lengthen the yard to hold 10 more cars.
Other than the rare Seaboard offerings, I doubt I'll get too many more cars. No room to store them and no room for them on the layout. I wouldn't mind getting a single car to represent each of the major RRs of the 50s-60s, but multiples of cars is out. Maybe cars for some of the southern RRs plus NYC and GN, but that's about it.
Of course the minute I say that's enough the market will be flooded with correct Seaboard cars 
and I'll have to rethink my limits, but for now I can honestly say there's nothing currently available that I desire, plus I've never been one to buy something just because it's new and available.
