Owen sent in this tip to share:

A snagging train coupler can also cause derailments. Unpolished couplers (particularly on new cars), can catch and bring about derailments.

It pays to clean away any rough edges and if necessary adjust the couplers so they center properly. Some coupler brands and particularly older couplers are hopelessly uneven in heights. The key is to have then set at the same height where possible. And, it is a good idea to stick with one brand so you become familiar with how they perform. Mixing brands like Hornby and Bachmann can sometimes be problematic.