[aigel]

Lots of things went wrong here. The article says that they think the dog was not a service dog. A service dog likely would have made the retreat. And pet dogs are not allowed on BART except in a kennel.
The train operator is present for exactly this reason. How could they look down the side of the train and not see the person?
As far as improvements go, I never understood the totally unattended trains and stations BART has. It isn't a wonder the system is the wild west and a total dump. A conductor on each train would go a long way. I read something about "ambassadors" on BART trains as a new thing, hopefully that's a step in the right direction.

Tethering the dog to you is pretty common, but you have to be careful where you do it. In the field or open walkway, no problem. But when on a bike or in traffic etc., keep it where you can let go of the dog. Better it than you getting hurt.