Box car floor hieght

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.

I encountered the 3'-9" dimension (car floor/platform?), but I cannot recall the reference, or the relation to the situation on springs.  BNSF has modern (100T capy/286,000# on rail) box cars with a 3'-8" floor with the car light (not loaded).  Normally the springs will settle 2" under rated load, giving a 3'-6" floor height.  In motion loaded, the flexing of the coil spring groups would be a 4" range-- 2" additional downward compression to 2" upward unweighting under track irregularities.  This relates to a discussion the AREA (as it is now, although not then) had in 1901-1902 concerning standardization of box cars, as follows:

Most of the existing box cars at that time were of mainly wood underframe construction, as is your Labelle car.  There were however some advanced designs using steel underframes and end sills, to address certain problems, and it was seen that this would be the future-- namely, that what was then called the "low truck" would become universally used one day.  In those low-truck wooden cars, a floor height of 3'-6" was used.  (I did not look into the question of spring motion, but I suspect from the coupler centerline height (34-1/2") being the same then as now, this is the unloaded floor height.)  But to use the low truck in a wooden underframe, the wooden end sill had to be substantially notched; this was thought by most car builders at the time to be very undesirable (in contrast to notching a steel end sill).

So mostly the "standard truck" was used.  To this corresponded a floor height of 4'-0".  Additionally, if the car was refrigerated, the floor was raised nearly an additional 6" (to say 4'-6") (probably 4'-5-5/8", but I didn't have a readable figure).

You'll have to judge from whether you are notching the end sill if a low truck is involved.  Unfortunately I have no idea if the Labelle archbar truck is a standard truck or a low truck, or if there were two types of archbars.  I'd guess that their cast sideframe trucks are low trucks, but of course I do not know the exact era or truck type for your car.  Hopefully this information will guide you as to what to look for, if you can find a picture of the prototype.

I'd follow Jim's advice on platforms-- I was concerned only with inter-track and bridge clearances (side and overhead clearances), which had to be met with no more than 3" to spare.  Hence I knew a lot of this to the inch or even the fraction, since it was agreed the railroads would get enough room to upgrade to their current standards.  Superelevation effects and sway as well, but not platforms.

--Frank

To Jim and Frank, a big "Thank you!" for your interesting and informational answers.

I now have the information I need.