When in service mode, I would find it very interesting if all the services I have previously driven replaced the standard AI services. For example: If on NTP my Class 101 service from Manchester to Leeds was severely delayed - I had forgotten the AWS a few times meaning I had several emergency stops. Then when I next played on NTP - this time on a Class 45 from Manchester to Leeds the signals were red due to the previous train holding up the trains. This would be very interesting as it would make it that your previous gameplay affects the later gameplay. I hope you understand what I am trying to say.
For some people this would make the game unplayable very quickly.. Also it would be impossible to test it reasonably. I would more like it when the AI services would sometimes have random delays of a few minutes. So that i actually encounter a red signal once in a while.. (That isn't from a blocked up service that's never going to fix itself..)