Railtrack was the private body which owned Britian's railway infrastructure immediately after fragmentation.
The government, in the form of then-Secretary of State for Transport, Stephen Byers, put Railtrack into administration in October 2001. The assets were subsequently transferred to Network Rail.
Railtrack is undergoing liquidation.
There is still an outstanding court case over the legality of Byers's administration order.
Further Reading
Railtrack liquidation site
Transport Blog commentaries on Railtrack
Transport Blog commentaries on Railtrack's administration
Transport Blog In Brief items
A Short Note On The Structure Of The UK Railway