Did you check the article discussing the timetable? It says that the nuclear layer is a future layer that will become available upon release of Skyhooks cargo line nuclear dlc which does not yet have a release date. Once that dlc is released and purchased, assuming you have all of the other dlc needed, you’ll be able to activate that layer.
Technically the flask train would not run through the area as the furthest south nuclear power station is Hinkley Point near Bridgwater. There is very occasional movement of spent submarine fuel rods from Devonport but these are (or were) highly classified and I wager unlikely to make it into the sim.
