RE inbound feed, a few websites have the ratio of feed at ~1.3 lbs of feed to 1 lb of salmon... assumed a 263K GWR, that would be 95 tons per car, or 274 cars/yr. Now exactly what salmon feed consists of, where it can be made and how it can be shipped is another story.
On the outbound side, if I could refine Smitty's comments... I understand the point of Smitty's in-state load limits, but this fish would be moving out of state, so would be loaded to 80K GVW. (And Maine's weight laws require a third trailer axle - I'm guessing there are no tri-x reefers domiciled in Maine.) That said, you'd get just shy of 900 truckloads/yr. Translated to reefer boxcar, the ratio is roughly between 3.5 and 3.8:1 UP's newest 72-ft cars have a tare of about 105,000 lbs... 64-ft reefers have a tare of about 92,000 lbs. Assuming 263K GWR, that would allow a 158 - 171K load, or 225 - 250 rail loads/yr, which ironically is about the same end result.
I'd guess that TOFC reefer would be more viable than rail reefer unless there is equipment inbounded to Maine that's available for an outbound load.