I bought a 6424 a few years back with a couple of repro cars, which are slightly shorter than the real deal, so I'm agreeing with CW on that. The best way to start your research is to (fun!) get a 6414 with original cars - from someone you can trust. Use these as the baseline for other cars (again, to CW's point). I love the auto carriers, but WILL NOT purchase or bid on anything that has a "funky" colored car, i.e., olive or lime green, unless I can get it really cheap and like the weird cars. What are other "funky" colors? Here's the fun part - research the web and ask around. Those repro cars I mentioned are gorgeous. They're a dark red (a funky color), have the Lionel logo on the bottom and are a couple of millimeters shorter than the real thing - it's obvious they're not a rare, fund-my-retirement variation. They're also well made. The guy I bought it from did NOT misrepresent them, either. I think I gave $15. Oh, nice, new looking tires can be a give away. I look for faded tires with no sheen; metal axles old and weathered looking, maybe a little rust. Have fun!!
Carlton