3'9" to 4' above rail head. Also platforms should be no closer than 5'6" from the track center line. Some railroads may have different specification bot most follow these.
Some shippers made their docks slightly higher than the 3'9" standard for box car floor above rail head. This was to facilitate"down hill" loading. Similarly companies that were primarily receivers of good made their dock heights slightly lower for down hill unloading.
Dock Plates (also called dock boards) were ramps that spanned the short distance from the dock to the car floor.