they open it with just the front rows and progressively move back to full trains once it's warmed up. It reduces the chance of a rollback if all the weight is at the front.
If Dragster's PLC is anything like Xcelerator's, it takes the average weight of every 5 launches to determine how much power is needed to get the train up to the 79.5mph that is required for a train to clear the top hat. I remember back when I worked Xcelerator a short shot was guaranteed if you completely filled a train early in the morning. Same was to be said at the end of the day. We had to slowly close off 1 row at a time for the last 10 trains or so of the day to prevent a "Catch car overtravel" fault when you sent the 1st empty train to bring the last train of guests back into the station.