I think I should point out that we definitely do not recommend building your route in the session layer...
There's two reasons for this.
The first is that your route/scenery will not be in the route itself, hence if you create a new session, you end up with items missing from the route. Click on 'edit route' and you'll see what I mean.
The second is due to a known, but currently unresolved, bug in Trainz that will cause junction levers (on junctions built in the session layer) to disappear if you 'resume' a session (save within Driver, then resume later).
It's possible that there may also be other issues in building your route in the session layer that are not yet known, haven't been reported (or pinpointed to using the session layer). Doing this hasn't been tested as extensively as, say, a route built in the route layer, as it's not something that we want to see route builders do.
That's not to say session specific items shouldn't be in the session layer. For example, additional signals or speed signs, or even extra track if absolutely required...
Regards