They market the design as something that can carry more passengers but that also means there is no way to secure a metal door between you and a knife-wielding attacker.
I also question the wisdom of a design that where, in a crash, hundreds to thousands of people can be thrown forward and crush people in the front car instead of having separate walls to limit how many people can crush those in front of them.
It would also be an issue where one homeless person would stink up the entire train.
Actually there are sliding doors between the cars. I have visited Toronto and rode in their subway.