The dimensions and location were later changed in 1920.
The dimension became 7/8ths of an inch for a rod or 1-1/4in pipe. A mininmum clearence of 2-1/2 inches was added.
The location was changed to not less than 36 inches and not more than 54 inches above platform or running-board.
These newer specs can be found in Appendix A of the _United States Safety Appliances for All Classes of Cars and Locomotives_ book issued by the AAR Mechanical Div.
"Given that the handrails were safety appliances, was this height used by AC&F based on a rule or regulation?"
The United States Safety Appliance Standards for tank car safety-railings:
Number: One (1) continuous safety-railing running around sides and ends of tank, securely fastened to tank or tank-bands at ends and sides of tank; or two (2) running full length of tank at sides of car supported by posts.
Dimensions: Not less than Three-fourths (3/4) of an inch, iron.
Location: Running full length of tank either at side supported by posts or securely fastened to tank or tank bands, not less than thirty (30) nor more than sixty (60) inches above platform.
Manner of Application: Safety-railings shall be securely fastened to tank-body, tank-bands or posts.