Rob,
The cars are held against the "Region" for a route. I can't remember in TS2009 but in TS2010 Surveyor if you go to "Edit Route" you can change the "Region" to one more suitable for your needs. If you can do this in TS2009 then it should solve your problem.
Otherwise it comes down to a direct edit of the config.txt file which is a bit more complex. Beyond this you can, if you wish, define your own region and set exactly what cars you want (by picking them individually) - this gets even more complex to explain than it is to do!