First
developed in the early 80's, the Twinstack was the first double stack
car to become popular with a number railroads. The five unit articula tion
concept was used to reduce weight and coupler slack. Why five units?
Santa Fe had used 10 unit spine cars since the
seventies but since one bad section bad-ordered the whole set, 10
units meant the car was out of service too much.
Five units provided the perfect ratio of cost savings to down time.
The bottom position of each well will accommodate one 40' container
or two 20' containers. Some railroads ordered their cars to handle
20 footers only on the two end wells while other railroads opted  for
20 footers on the only the middle three wells. The difference lies
in whether "hard points" were attached in the
middle of the well to support the ends of the 20' containers. The
upper position will hold 40', 45', or 48' containers. Twinstacks belonging
to SeaLand had extra attachment points so that company's proprietary
35' containers could be carried in either position. The large bulkheads
at the ends of each well were designed to hold the top container in
place.  Even
these large structures were sometimes not enough in high winds as
empty containers were known to blow off. To stop this rash of un-piloted
flying cargo boxes, some cars were equipped with 8' high walls that
were welded to the tops of
the bulkhead. Hoping this would deflect the problem with the
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