Firstly, we're not 'replacing HTTP'. HTTP runs over Railgun in the same way that it does over SPDY and the content servers are still running HTTP.

The big benefit is not in terms of bandwidth saved (for us) it's in terms of total time to get the page. That's partly driven by latency and partly by bandwidth. Because we have worldwide data centers we can see high latency from say the data center in Miami and a web server located in Sydney. Railgun helps with that problem.

Also, CNN has a TTL of 60s but many, many web sites have a TTL of 0 because they want no caching at all (see New York Times web site) or because the page is totally personalized.