What did you do to your trains today?

I haven't heard of the "9V battery trick", so I'd like to know what that is and how a 9V battery would fit. I just happened to find a good working motor from something totally different, which should be good for my first motor swap attempt. However, it might be better to wait until I could start using the models again so as to avoid killing that motor as well. I'd better check the other engines to make sure it only happened to certain ones.