How did the train robbers know?

So in the first robbery the robbers set up a roadblock to stop the train. There just happened to be an open-air train car which stopped above the bridge. The robbers conveniently parked a truck under the bridge, unloaded specific boxes of ammunition (and smoke pipes) from the open-air train car into the truck and left.

Sorry if this is a dumb question, but how was all of that so convenient for the robbers? Did they have an inside man who informed the gang of the train's contents ahead of time?

On a side note, it seemed silly to me that when the investigation was starting, the investigators seemed puzzled that a box of smoke pipes was taken from the train. I said, "Well, it's not like a trip to the grocery store, where you usually have time to be choosy about what you take with you!"

Mag, Darling, you're being a bore.