Our inspection pit in the enginehouse at Long Leaf is only 3' deep as well and I wondered when it was restored why it was so shallow. It is a real pain when working on our rail car and motor cars because of its shallow depth, but after I got to thinking about it and seeing how locomotive work is done when you have deeper pit, I think that I understand now. In a deeper pit (5' or 6'), everything is above your head and if you want to work on it or do a close inspection of something in the running gear, you need a ladder and have to move it around in the pit, and it is both an obstacle and extra work. In a shallow pit, sure it is a pain to get around in, but then everything is at eye level, or just slightly above or below eye level when you stand up and get your head inside the frame. If you are 5' 6" tall (the average height of a man in the 1880's, and you stand in a 3' pit, your eye level is 27" above the top of the rail. In a NG loco, that means that you look down on the top of the axles, directly at the axle boxes and the spring rigging in and on top of the frame, the brake rigging and everything else is at eye level or slightly below. Tools don't have to be carted up and down a ladder to work on things, just reach down and pick them up. In a SG pit, like ours at Long Leaf, that puts you just at eye height for a locomotive axle with 50" drivers which was typical for freight engines in the late 1800's and early 1900's.
My thoughts, and more than $1 will get you a cup of coffee these days. It probably made more sense when coffee was just a nickel.