It's hard to imagine how a 15-second delay holding the doors would have effected the departure time. Feels there's more to this.
I’m the one who tweeted that stuff, when the doors detect a blockage they try to close again 3 more times, each time getting progressively harder to try and force anything blocking the door out of the way, after the 3rd time the door must be manually cut out and back in (or just remain cut out) by a crew member. (C):
That took 5 minutes, the other 6 minutes were waiting to leave Union as the train now had to wait for other trains in the USRC to make way.
This Aurora-bound train had to be cancelled at Maple because this train deadheads from Aurora to Kirby siding to make way for the southbound Allandale train.
The southbound Allandale train has to pass a northbound Allandale train later down the line at I believe Concord. After that, the northbound Allandale train has to pass that deadheaded Kirby train before the train at Kirby can go back to Aurora for its southbound trip.
With a 11 minute delay this train would be getting to Aurora after the Northbound was scheduled to depart, so it had to be cut back.
Delays on the Barrie Line cascade very quickly and short turning Aurora trains at Maple avoids serious delays to other trains to and from Allandale, if those 2 trains are delayed it proceeds to delay the next 2 northbound and southbound Aurora trains, and the problem continues.
Shuttle buses were quickly dispatched and communication was clear.
The engineer or conductor also came over the PA system and told off whoever held the doors, saying something along the lines of “I’m happy you made the decision that you wanted to to sit here and hang out with all of us as we fix the problem you made”.