All franchise agreements let from 2003 onwards have revenue support/revenue share clauses in Schedule 8 of the National Rail Franchise Terms. The Train Operating Companies with these provisions are:
Cross Country
East Midland Trains
First Capital Connect
First Great Western
London Midland
London Southeastern Railway
National Express East Anglia
National Express East Coast
Northern Rail
Virgin West Coast.
This list will include the new Southern Franchise when it begins on 20 September 2009.
Full details of revenue support/revenue share can be found on the Department for Transport's website's Public Register of Franchise Agreements:
http://www.dft.gov.uk/pgr/rail/passenger/publicregister/