I have researched the Railguns technology, but I cannot add a Railgun to a new ship design.  I can add an Enhanced Railgun, but I cannot add a plain-old Railgun.

Here is a link to a save file that should allow you to recreate this issue:

https://www.dropbox.com/s/np0ga1u6mukekur/Railgun%20Issue%20Under%201.55.376894.1.GC4Sav?dl=0

After loading the save file follow these steps to recreate the issue:

1. Double-click on either of my shipyards
2. Click on the "New Design" button in the lower right-hand corner
3. Select "Fighter" class then click on the "Start New Design" button
4. Choose any hull (I use the "Yor_Sniper_01T")
5. Click on "Equip" then select the Weapons icon
6. Click on "Railgun" then try to place it anywhere on the ship - you can't!
7. click on any other weapon then try to place it anywhere on the ship - you can!

You might also use this save file to explore why Culture Points are so hard to come by.