The trains don't reach their Top Speed when connected with cargo or fluid wagons. It's working when the Space Locomotive or the Maglev Locomotive are without any other wagons.
Their speed is capped at 372,6 km/h somehow. But their theoretical max speed with nuclear fuel should be 745 km/h (648+97).
I'm using the Space Exploration mod with some other minor QoL mods (like squeak through, task list, etc...).
I really like the trains and it would be really cool if they would go to their max speed.

Thank's for reporting, I'm going to look into it

I've just published a hotfix that should resolve the problem. Keep in mind that if you are using a Maglev and Space Lokomotive on that same train, the speed will be limited to that of the space locomotive (space wagons are not limited), the same goes for base-game cargo wagons/locomotives