They have never directly said it's not on rails. They have said over many interviews that
1. current space combat was just a place holder for launch
2. the SSSP is something completely new and expends space combat in major way
3. the SSSP is a completely new pixel space
4. that the current space combat has nothing to do with the new SSSP
taking in all of those statements it's clear that it would be foolish to assume that it IS on rails.
now how big it is or what it entails who knows.
Links to those quotes please