Handling grain tends to need specialist equipment and people trained to use it.
You can from memory use basically bulk "dry" containers to ship it, but you need to have things like grain "elevators" and conveyor belts to quickly and safely move it from say a truck to the rail cart, then similar at the other end.
With rail transport one of the ways they do it is special cars that are designed so you load into the top, then at the other end the car goes over a raised pit and the bottom of the car opens to empty it out quickly using gravity to do the work.
However you need hundreds/thousands of rail cars to move what you can do in a single ship load.
And all the time you're doing this you need to monitor and control the dust levels, make sure everything is earthed properly to lower the risk of an explosion (scarily common if you don't handle it right*), whilst trying to keep it dry and preventing rot or contamination.
In a time of war you've got the problem that not only are you contending with say the other side trying to destroy your transport facilities, but you're already juggling the priorities of what gets sent, when, and how, and the disruption, so military supplies will get priority, and you may not be able to get the train cars needed to be moving back and forth efficiently or at all.
*the grain dust can go up like a bomb if there is the right mix of dust and air and you get a spark.
