Ken’s comments are right on but something you should also check out is are your cars too light? Most manufacturers skip on the weight requirements (eg, they’re too light) but this is an easy fix by adding weight(s) until they meet NMRA standards.

Since you didn’t mention what scale you are in, I suggest that you go to this web link to get the specs you should be using:

http://www.nmra.org/standards/standards.html

Look up your scale’s specs and see what weights you should be running (it varies by car length). You can pick up a cheapo digital scale on line, at eBay or at any store that sells kitchen equipment for about 10 or 15 bucks.

Hope this helps,

Geof

PS The NMRA track gauge that Ken mentions can also be used to make sure your track’s gauge width is correct as well. Pay attention to switches (turnouts) which are usually common trouble spots.

You also may have ‘pizza cutter’ flanges on your wheel sets. These are wheel flanges that are very thin and ‘sharp’. They can ‘split a switch’ at the points with no effort at all. If your derailing cars have these type of wheels/trucks, bite the bullet and replace them with better quality trucks (like Kadees if you are running HO gauge).