Would you believe there WAS a system for the Pennsy numbering? There was! The PRR used numbers by divisions. There was no rhyme or reason as far as wheel arrangement or class as far as numbering went, but there was division assignment. For example, PRR 10 was an A5 0-4-0, 11 was an L1 Mikdao 2-8-2, 12 a K4 4-6-2 Pacific, 13 was an E6s Atlantic 4-4-2, and PRR 14 an L1 Mike. These were all on the Chesapeake? or Atlantic? Division (new Jersey, Delaware, Baltimore, Washington D.C.) And like was previously stated, when these locos were retired the numbers would be reused by new engines assigned to said division.
PRR did finally begin using a numbering system by class with the introduction of the M1 class 4-8-2s in the 6700-6800 class.
