"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.