This car was lettered "UNITED STATES ARSENAL WATERVLIET NY No.1" and rode on four 3 axle "buckeye" style trucks. The flat appears to be approximately 70 feet in length and was built to carry large guns (ie 16" coast defense artillery).
It was built by Magor in 1918, and there is a 3/4 builder's photo on p.79 in the Magor book. This car either supplemented or superseded D&H 50000 and 50001, a contraption consisting of two shorty 12-wheel flats built by PSC in 1916 with a gun carriage bridge built by D&H.
