Although this method is not the best to soundproof your Brio from outside noise, but some people here says this method really work, and more importantly cheaper.
Original DIY by Danny Gamadhi
What you need:
Aquarium Air Line Tubing
The idea behind this DIY, is to make your door tighter by inserting aquarium air line tubing into the tunnels of the rubber door stripping.
You need to find the opening at the edge of rubber door stripping. And you can start inserting from that opening.
To make air line tubing slide more easily, spray some aerosol silicone.