The axle weight of the Triplexes blew everything else out of the water. Even the BMT standards didn't come close.
Not necessarily. According to the literature of the time the IRT motors had roughly comparable axle loads. Moreover, the maximum axle load for any rolling stock was limited by 31,000 lbs, not figure of 40,000 lbs.
That said, the Triplexes could have been used exclusively on underground routes and operated via tunnel.
Interestingly, the Triplexes were supposed to be used in the Tunnel, when they were purchased, if one were to believe the articles in the Transit Journal from that time.