My new idea concerns pools. If you simply created a way to have a pool of cars (as I stated in another post around here somewhere), freight or passenger, then you pull the engines from the engine pool and the cars from the car pool and go about your business. The game wouldn't have to deal with the traditional consists. The pools would be the default way Timetables kept up with trains, while the old consist file (or the new OR equivalent) would take care of Activity Mode. All of this could be put into a "Consist Editor". You can either create a consist, or a pool depending on whether you're working on an activity or a timetable.
At any rate, there does need to be an ability to create pools within the Timetable Editor even if it isn't part of a Consist Editor. Some kind of tie-in to the Track Viewer in both of these editors would be helpful, too.