From the defender’s point of view:
If you throw airstrike to the left, you can wipe out the entire enemy team. Not to mention that you can place turrets in a multitude of ‘safe’ corners to make pushing through as hard as possible. Not to mention that if they cannot destroy the gate, it means they cannot hide within the buildings on the left side.
As for the right side, there is a MG greeting that entry. Park it with an Aura with the overheat reduction augment, and the enemy will struggle to push through. Plus someone else to make sure that no one flanks the Aura. If the right side is defended, the attackers cannot push themselves to the right side building, and through tunnels to the left side buildings.
Thus the when the defence on the blocker carriage fails, it is often due to the defenders ignoring one side of the map. Allowing attackers to keep the defenders at bay, while they destroy the door controls, and plant the C4.
Well, it is possible for the blocker carriage defence to fail if the track controls are destroyed with the defenders and they must fait for 23+ seconds to spawn, allowing the attackers to plant their bombs and get ready to defend.
Now, what is more like a mission impossible is the last segment of the map. In other words when the data cores are to be delivered, and the last remaining datacore slot is facing the defender’s spawn. It is one of the best turtle defence positions in this game. (Well, the other being the sample room in Bridge.)