A weed weasel is a railroad official usually a trainmaster or other equivalent manger. Its not just an SP thing, its nation wide on every railroad. Weed weasel is just one of many names used to describe them.

What most railfans don't know is that railroad mangers are required to do operations testing(spy)on train crews. For non-railroaders working in an office environment think of it like your boss hiding behind the ficus with binoculars watching your every move. They also do set-up tests to see how train crews react.