General Talk: For anything you want to talk about including your railroad
Narrow Gauge: If you model narrow gauge in any scale
Large Scale: For all of the G scale and most O scale
HO Scale: For HO scale
N and Z: For N and Z scales
Micro layouts: For very small layouts in any scale
Photography: For any scale, any layout size, really anything you took a picture of that is train related.
To be honest, we have many options on where to post about a particular topic. For example, I model Fn3 (gauge 1) trains, so any time I post something, it could be in Large Scale, Narrow Gauge, Photography, General Talk, or Logging and mining.
If you need advice ask one of the head honchos through a PM, someone will get an answer to you within a resonable amount of time.