It is not entirely clear what is happening.
Network Rail is the successor to Railtrack, which was a public limited company with shareholders.
Network Rail has no shareholders, not even the government. Many people think of it as in fact a government owned company but its structure is a "Company Limited by Guarantee" - in a way, a sort of cooperative. My feeling is that this won't last.
The reason it is not openly a government compay is said to be the desire of the Treasury not to carry its debts on the Public Sector Borrowing Requirement. But that is really a fiddle.
Now NR is to take over most of the functions of the former Strategic Rail Authority. It is beginning to look a bit like the former British Rail. However the operators remain private companies, and the providers of rolling stock will continue to be private leasing compaies, mostly owned by banks. NR is taking the maintenance in house which means far fewer contractors than there have been since privatisation. It is hoiped that costs will come down and the quality of work improve.