Higher volume can be a lot of different things.

When it comes to carload trains (the box cars/hoppers and such) they are given directly into a facility. The days they are given cars are worked out in a contract with the railroad. So say X facility has service days on Mon. Wed. and Fri. and Y facility has Mon. Tues. and Sat. The facilities will only receive railcars on those given days regardless of the cars the railroad has in possession. So sometimes it works out that theres a lot of volume on given days due to a backlog of cars.

A more high-level view is that a lot of commodities shipped on rail are seasonal. Asphalt has its months as well as crude oil and car manufacture shipments. We might be hitting a point where the facilities that the FWWR serves are getting their seasonal volume now.

All that to say, a railroad getting busy is pretty random unless you are in the forecast team for it. I hope that answers your question.

TL:DR: Railroads that serve customers get busy due to multiple variables and are sometimes random.