This has happened two me twice now in less than two months. Trainz has saved a blank session over my carefully set up schedule with around 25 interacting trains. The first time I assumed I had loaded up the layout without the session, and saved that (and what sort of design decision was it to make that so easy?). Since then I have carefully checked there is some rolling stock around before I save so I know I loaded the session (specifically, I saw a class 304 EMU, two minerral wagons and a brake van; all disappeared when I went into driver mode). This is not a bad design decision and me being careless, this is the software screwing up.