I’ve taken my cat to the vet on the bus and metro. The carrier looks somewhat like a mesh gym bag and the cat was quiet, so no one noticed at all. I had a printout of the page from Metro’s website saying it was ok, just in case someone raised a question. I have seen people bring small dogs (<10 lbs) on metro in a backpack like carrier with mesh on the sides and top. I think as long as the animal is quiet you shouldn’t have any problems.