Anyone who has ever tried to keep fish alive, be it in an aquarium or simply a live well on a boat, knows how problematic this can be. Fish require oxygenated water, which would be difficult to maintain in a rail car and virtually impossible in a barrel aboard a ship. I imagine you might be able to keep a single cod alive in a barrel for a day or two, maybe somewhat longer, but not for a sail around the Cape. Similarly, a rail car might maintain a dozen or two full sized cod for a trip across the country. Since dissolved oxygen is inversely proportional to temperature, the venture may be better attempted in winter than summer. Artificial aeration would increase the number of cod, but not dramatically, while adding to the expense incurred.
Either way, it seems impossible to transport an economically viable number of live fish over such a distance by any stated method. You could move a few, surely, but they’d be worth more than gold by the time they reached their destination.
The only realistic method that suggests itself would be a ship equipped with a live well of enormous size, and capable of pumping raw sea water through it constantly, to maintain oxygen levels. I’m pretty confident this would work, but still, the economics seem unlikely to be acceptable.
Wild tuna are caught today and “transported” great distances across the ocean in giant net cages. Oxygen flows in, waste flows out, and the fish remain (usually) alive. They may also be fed and grown to larger size in these fish pens. But this is tuna, highly desirable and commanding premium prices, not cod.
So I question the basic premise of this tale as being economically unsound. Adding catfish would only raise the oxygen requirement or reduce the number of cod that could be kept alive. The whole thing sounds like a non-starter. I’d be very interested to see documentation of any actual attempts in this enterprise. If there were any, I’m betting that they were one-off abysmal failures.