Not sure about trains, but large ocean-faring nuclear-powered tugboats have been considered to move container ship sized barges most of the way and then let battery or fossil powered tugs bring em the final mile into port.

https://onlinelibrary.wiley.com/doi/10.1111/j.1559-3584.1963...

The cost of labor is too low, and the prevalence of pirates too high, to justify this idea. You're also talking about serious intermodal requirements to handle these things when they get to the interface between open ocean and restricted waters, because safe navigation is a near impossibility with the tug configuration you're proposing, especially once navigational constraints (e.g. land or shoal water) come into the picture. A large merchant vessel takes several miles to stop on its own, before you consider any towing configuration. If you've never operated around them, you probably don't have the instinct that you can be in serious danger of a collision while still several miles away from another vessel, but what you're proposing is scary.

Even without the navigational problems, you're necessitating armed guards by having these ships operate internationally, because these ships would be huge pirate targets on a much more severe level than the scrap metal and parts theft that is present throughout the world. That gets... tricky, legally (see https://www.swedishclub.com/upload/Loss_Prev_Docs/Piracy/PIR...)

Also, cargo ships are already enormous. Trains pale in comparison. We don't need these headaches. Oceangoing shipping is miraculously cheap already.

Edit: I'll also point out that, regarding the sibling comment's link, oceangoing shipping was a much smaller club in 1963, so the navigational challenges would not have seemed so daunting. Containerized shipping didn't even exist until the mid-50s, and AIS and radar let us safely pack way more vessels into small spaces than they could have suspected.

The feasibility of convincing enough countries/port authorities to permit you park a nuclear reactor in their port is fairly low. If something goes wrong, they'll be the ones that have to clean up the mess after your shipping corp declares bankruptcy or otherwise skips out on the cleanup bill.