I've added the nuclear locomotive as upgrade from either express or heavy locomotive.
It's stats are slightly below the specialized variants on solid fuel and noticeably lower than running them on rocket fuel so it doesn't become the perfect locomotive in any situation.

It also explodes taking a good portion of your base with it when destroyed. You may not want to use it in biter infested area, or especially there to clean them out with suicide attacks. ;)

If you'd rather have the specialized versions each having a nuclear refit into heavy nuclear and express nuclear locomotive let me know here.

I have not played with the nuclear locomotives enough yet to really say, but it seems like it could be neat to have specialized versions. I did modify the nuclear locomotives to have 8x4 equipment grid (for bob's equipments), and the heavy/express to 8x3, and regular to 8x2.

To represent improved tech, and to give the nuclear locomotive a bit of edge out in the wildness (add shielding/self-defense if needed). They will still go boom if they get stalled.

For now I'm going to keep a single version of nuclear locomotives. Hardly anyone would use a version capped ~200km/h when there's a 300km/h with only marginally slower acceleration.

Nuclear locomotives have a considerable health boost for survivability. I didn't add grids directly since Vehicle Grid (https://mods.factorio.com/mods/Optera/VehicleGrid) does so anyways.

Can you add a Mod Options tab to disable the nuclear explosion in-game when destroyed?