It is possible to reset the session rules (all of them or only selected ones, depending on your needs) in TRS19PE only. Not in TRS19.
TRS19PE was the first version of Trainz to support the UDS which is now found in Trainz Plus and TRS22. The reset buttons in the Session Editor is used by the UDS in TRS19PE, Trainz Plus and TRS22 (all versions).