Rail figures are harder to find, but a reasonable guess based on what I can find seems to be 400 containers per train if the loading gauge permits double stacking containers. Speeds over 100km/h are easily achievable on railways without at-grade crossings.
So one ship could easily require ten trains to move all its cargo in the same time, plus the time taken to offload at each end, which I guess is why this hasn't taken off. Though if it's all just transhipped to trains this could be faster than at most ports where containers have to be sorted to different destinations.
Though interestingly with a dedicated right-of-way the trains don't actually need to be manned, remote control freight trains exist.