Pacers have a "L" shaped doors system. Two doors at each cab end, but only one door (per carriage) at the other end of the coach (e.g. middle of the unit). So everyone seated in the middle of the train has to use the one door.
See this pic here:
http://www.flickr.com/photos/thebustocrookes/5261594836/ - the front coach has a door at the "cab" end and a door at the "middle" end. The second coach has only a door at the cab. On the platform side, the "middle" door is on the second coach.
Or this pic:
http://www.flickr.com/photos/thebustocrookes/4481961105/ -only the right hand coach has a "middle" door - on the other side of the train the "middle" door is on the left hand coach
[Door Door]
[...............]
[...............] << Front Coach
[...............]
[.........Door]
--------------
[Door....... ]
[...............]
[...............] << Rear Coach
[...............]
[Door Door]
(if that makes any sense)