Removing 'unused' assets is a big risk at this point in time.
Textures, especially, do not get recorded correctly as dependencies in routes/sessions.
In addition to this, rules and driver commands are not normally recorded as dependencies due to a Trainz bug.
Trains that are emitted from a portal are also not recorded as dependencies.
I will be looking into sorting out a utility to help out with unused assets, but I need N3V to sort out the dependency recording bugs (which I have a ticket relating to with the Helpdesk) first.
Shane