In my perfect hindsight 20/20 contingency plan, I would hold the first train at Bramalea to accommodate the remaining passengers, then let it fill up as the following trains arrive, dump passengers, and turn back east. The unknown is how long it takes to fill up completely (and how many people can grab a bus, etc). The other question would be how many trains have to hold because they need to tie up at the outer ends. There is enough platform capacity at Bramalea for a 'shelter' train plus the turnbacks, but that may not be the case at all stations.
Under no (avoidable) circumstance should a GO train hold between stations for an hour or more with passengers on board.... a health and wellbeing issue. But I could buy one staging at Malton, the next at Etobicoke North, etc once Bramalea is full.
Definitely something that should be planned out ahead of time, and not attempted on the fly.
- Paul