on Wed Oct 11 17:03:04 2017, in response to Re: Tuscarora Almanac for October 10, posted by W.B. on Tue Oct 10 22:06:19 2017.
What happened with the 4 Av Subway as well as the Lex Av IRT and the Pelham and Jerome extensions was that the PSC decided to construct all new subways to accommodate the larger RR commuter coaches which were becoming state of the art at the time. While all the lines that eventually became BRT/BMT lines were constructed that way during construction, the PSC realized that it would be of no use to have the new IRT lines constructed that way since they would be connected to the “first subway” with its more restrictive clearances where only the smaller IRT sized cars could be operated. As a result, while the 7 Av IRT was built slightly larger than the original subway, it wasn’t built to the larger clearances of the Lex which had been started earlier. I’m not sure what clearances the Bkln IRT was built to, although I suspect it might be to BMT dimensions as well. In any case, once BRT management became aware of the clearances its new lines were being built to, they designed the largest subway car that could be practicably operated there and so came up with the 67 X 10 ft design.