Although it also sounds like Jenna’s voice in the few lines we hear in the most recent Halloween episode, we are 99% positive that it was Mona who was trying to push Aria and Garrett off of the train with who we think is Jason. 

Garrett was stripped of his costume, the costume that was found with Mona’s stuff in the custodian’s office.

 Considering that Mona and Jason are pretty close, this might prove that Jason and Mona were the two people that were trying to push the box off the train! It might be also because Mona just wants to kill him like the others, that means 3 NAT members would be dead. Unsure of this yet, we’ll have to see more.

I guess we will just have to wait a little bit longer to see more!