Railgun is a web optimization technology which is provided by Cloudflare that speeds up the delivery of non-cached web pages.
The basic Cloudflare service automatically caches nearly 65% of your web pages but, 35% of the content remains non-cached (which is often an HTML web page content that is downloaded first).
Railgun than serves this 35% of web page content through a secure network between the Cloudflare network and your host’ origin server. Thus, this reduces the usage and overall website loading time.