Railgun is a wide area network (WAN) optimization technology. Unlike plain CloudFlare, Railgun uses alternative compression techniques to speed up the performance of non-cached site pages and ensures that the connection between our web server and the CloudFlare CDN is as fast as possible.
Before proceeding, please make sure CloudFlare is enabled for your website per guidelines here.
1. In the CloudFlare menu select the Performance option: 
2. Scroll down and click on Settings next to the CloudFlare-powered website in question. Select Railgun from the drop-down and hit On: 
There is no Railgun in Settings menu
The Railgun service with CloudFlare can be used only for domain names that are pointed to our hosting nameservers and resolve to our server IP.
That's it!
Need any help? Contact our HelpDesk