The successful terrorist attack of 9/11/2011 brought home the valid point that ceding control of the cockpit willingly or not, was no longer a valid response to a hijacking. Instead, it would now be paramount that maintaining control of the cockpit was not only essential for the safety of all, but that it was also mandatory in order to make any nefarious plans for attacking the cockpit, an exercise in futility. The response within America after 9/11 was to increase the security for the cockpit by better locking mechanisms, the usage of double-doors within the cockpit, the usage of security cameras and certain security protocols in conjunction with flight attendants, human security marshals on random flights, and a motorized Kevlar-type material that blocks the hallway to the cockpit. All of these items have helped considerably in maintaining control of the cockpit, along with the obvious foreknowledge that the pilots of these airlines have a responsibility and a duty to police their own domain.
While one can make several arguments of the superiority of one security device and/or procedures over another, it doesn't take any real stretch of the imagination to recognize that if the pilots never leave the sanctuary of their cockpit, or their secured cockpit area, and are never visited by any airline personnel whatsoever during the duration of a particular flight, that this provides the highest conceivable security possible, to which a double-door system appears to be the most useful. In fact, double doors can be readily designed through a system that provides a small area for a bathroom, first aid supplies, refreshments for the pilot crew, and at the same time absolutely minimizes the need to engage with anyone outside of the cockpit, except for reasons of pilot relief, incapacitation, or wing visual inspection.
Additionally, currently on American flights there are the monotonous pre-flight safety instructions which should be immediately augmented with something much more meaningful which is the responsibility that each passenger has to the whole of the aircraft, that is to say, in situations to which there appears to be something that is happening outside the bounds of normalcy, passengers have a responsibility and a duty to respond to this potential crisis, such as what occurred on United Flight# 93, or the underwear bomber crisis on Northwest Flight# 253. This additional potential help would in of itself, be of immense importance in the securing and the prevention of a hijacking, or a detonation, or some other mayhem on a flight.
The double-door concept or similar is the same type of safety that should be employed on other mass transit transportation drivers. Clearly, in situations to which all sorts of havoc can be created by the loss of control of a mass transportation vehicle, integral steps must be taken to reasonably preclude the success of any particular hijacking. I believe that it is fair to say, the greater the mass of the transportation device, the greater the measures that should be taken for good, solid security to protect the public and our commonwealth.