Choujin Locke

Both anime are set far into the future where some espers are causing trouble and the hero, who happens to be the most powerful esper in the universe, has to defeat them.  

These two anime can almost be termed space operas. They're both massive on action, heavy on violence and deep, very deep in space! They aren't like the current crop of anime sadly. 

The most powerful esper on the planet fighting enemies weaker than him 

well, look at the animation, both have in a big part supernatural content in the movie´s, and both ae abouth the freedom of ppl that are ´rejected´ by society because they have been considered an esper ( locke) or a mu ( terra e....) psychic humanlike creature´s. boht i think are truly space opera gems 

movies with supernatural as the main theme wether in outer space or just on earth, though x is more related to religion and folklore than locke, u can count on flashy fights! 

Though Locke is more of a space opera and Railgun is super-powered slice-of-life, they both have the idea that ESPer powers can be developed. In the Locke universe, the students train at Lady Kahn's school to become ESPers. There are no levels or variations like there is with Railgun. Also in Locke they students are trained to be used as a took for evil. It's rarely their choice to use their powers for the good. Railgun, each other students are trained because of the pursuit of science. Some do use their powers for evil, other's good.  

The whole super-old-but-looks-like-a-teen super powered psychic thing is definitely similar. Also, Locke and Quon are both nice guys who would rather help people that mass slaughter people with their immense psychic powers. 

These long, '80s space movies have things like unpredictable enemies, star traveling, extraterrestrial side-characters, and blasting fights (not space fights rather) in common. Also, both main characters are considered legendary in their part of the universe.