Picture describes it best.

Each car is in a position where if removed it could not be placed again as there are only 2 pieces of track under them. This is done by placing the car as far to the edge of 3 tracks as possible then mining the far piece of track. This is where I believe the glitch lies if this is a glitch, whether or not that piece of track should be mineable.
You can keep doing this as far as you want. All four of the inserters between each pair of cars will move items between the cars in either direction. This 4 inserter state is easily possible between just 2 cars by edge placing two trains towards each other, it is only extensible to many traincars by using the ability to mine away the far rail.