Hello there !
First, thanks for testing the game !
In order to place anything on the core, you have to increase the power grid. you can see its state on the core and on the batteries, by hovering the mouse .
Each node has a certain piwer grid weight, written next to its name in the build window. Railgun are pretty heavy, and costs 5 on the PG. By placing batteries, you should be able to place it !