Wright said that trainers were brought in to rile the bugs before the scene was shot to encourage more activity.
This is not the first time Snowpiercer has relied on live animals for its production.
When I spoke to Ruth actress Alison Wright in Season 1, she revealed that the crew actually used live cockroaches for that sabotage scene in the first-class dining car.
I doubt anyone planned for the rat to try and escape in the botched version of the scene, so that may have just been a bit of improv the rat was trying out.