If it's never defrosted, moved directly, and within the expiration date, it should be OK if done correctly. Sorry, I'm not sure about FDA regulations but you should be able to search for a checklist that a state health inspector would use. The FDA guidelines are probably pretty similar. It has to move directly from one place to the other and be covered during transport to prevent contamination.
The main danger is allowing it to defrost while moving. Once that happens, it should not be refrozen without cooking first. If not cooked in time, it has to be tossed. Failing to do so would be the danger and also a ding on any inspection. Since it likely was transported over the road, through distribution centers, and onto the train, it's just an additional leg if it stays frozen. Warehouses and distribution centers are designed to minimize spoilage during this movement. Train stations are not and the outside climate is going to have a huge impact on the window for that movement (5 minutes in LA summer sun vs. 30 minutes in Chicago winter).
That all being said, unless it's full of preservatives, there is a noticeable difference in quality/taste if temperature fluctuates, even while under the freeze point. Temperatures in different parts of the supply chain are planned to prevent too many spikes one way or the other.