It is a good question. My father has working for network rail for some time as an engineering manager and i have also worked for them in the past. As far as i am aware the revinue from tickets is divided up according to how many trains each operator provides and for tickets that cross a number of lines then tha revinue from that ticket is divided between the lines according the the length of the line and the average speed of the train along that line. In summary each operator is given a percentage of your ticket depending on how many trains they run on the lines u travel on and how quickly they get u to the destination.