Not just DB, even our neighbourhood has excessive amounts of dog crap over the weekends when owners have to walk their dogs and think picking up is only something helpers do.hosts a lot of such dog owners who think it's OK to use the walk away from poo approach when their dog hasn't used a pad.
Dogs / cats in specially designed bags should definitely be allowed. dogs walking sitting around trains is a no no anyway not manageable in crowded buses / trains.
I do own a tiny poodle and I have purchased bags / backpacks and have taken her on the mtr a few times. It boils down to pet owners acting responsible.
It's sad to see many dog owners not picking up after their dogs. And it's not limited to locals, I have seen many foreigners do it.
So finally MTR CORP will have to invest in manpower policing around stopping misuse of pet policy.