There was a very, very brief toy train scene on the 12/3 episode of Castle.
This was the Christmas episode. In one scene where Castle was attempting to convince Beckett to come to his apartment on Christmas Eve, he said something along the lines of "You have to see my train set."
At about the half way point of the show, they showed a toy train operating for a few seconds. I was viewing on cable via the free on demand service, so the the pause feature is limited and no ability to view in slow motion, but it appeared to be 3 rail track, so was probably O gauge. I couldn't determine any more details.
Very small exposure for the hobby. If you watch the show, the people who decorated Castle's apartment did a really nice job.