I rode those BMT D Type Cars every day when I started to work at Con Ed NY in 1960 (before they were scrapped 1964-1965).
If I missed a train, I used to count 12 "cars" going by the platform, but later realized that these trains consisted of four units each having three "triplex cars" with the same number, but lettered A,B,C (example 6044A, 6044B, 6044C ). Each three "car" section unit rode on four 4- wheel trucks. You could always walk between the A and B or B and C section of any unit through that swivel connector above the two middle trucks (which was reminiscent of my first Lionel set, a 1935 Flying Yankee Train given to me by my father's boss since his two sons were going to Princeton and were considered too old to be playing with toy trains, but that's another story). I had that set on the basement floor when my wife kicked it while doing laundry and after she had four daughters and no sons, I sold it for $50 at my first garage sale. Later I learned what $ they were going for at the first swap meets I attended at Dover High School, the early Greenberg shows, METCA and York TCA.
Yes I would like to see MTH also model the BMT D type "triplex" since at the time 121 units (363 "cars") co-existed with the BMT Standards and Q cars while there were only ten prototype cars of the R11/R34
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