Pretty close. I believe it stands for "Non-Narrow Gauge." I don't think it is required, but people put it there to alert the folks who browse the forum that the post within is a bit off topic. Based on the inputs we've gotten from the moderators previously, I don't think they mind a little NNG steam railroad content, but they've expressed a preference that we don't stray too far off topic. In other words, there are probably better venues for the diseasels, airplanes, dog & cat pictures, etc.
/Kevin