The resort monorail starts (I believe) at 7am. If you are trying to get on at the TTC though, you may have to prove that you have an ADR or show a roomkey to one of the monorail resorts (as there is a CM who checks) to board the resort monorail in the morning, otherwise the Express monorail generally seems to start 1/2 hour before the MK opening time. This was the case in May HTH