The new subway cars in Toronto have an excellent feature that I wish all transit agencies would adopt. The "open gangway" means that the cars are connected to each other in an articulated train set with a section similar to the middle "accordion" section of light rail cars and long buses. The ability to walk easily between cars increases capacity by allowing crowds to disperse throughout the train. It also improves safety as you can see through the entire train and won't be isolated, and nobody can become injured when trying to pass between cars on a typical subway train. This setup is apparently common on trains in Europe but not in North America.
The main disadvantage of having the cars permanently connected is that an entire train set must be removed from service when maintenance is needed, so you need more cars in your fleet. With a typical setup, one or two cars can be removed and the rest coupled with other cars ready to carry passengers. I think it's worth spending a little more money to get the benefits of articulated train sets. Although I guess it would also be bad if you can't change cars to escape the guy who hasn't showered in weeks.