Don't wanna start flames... but it's probably better to stay positive.
The idea of 'you *do* know the relationship between shutter speed and f-stop' should be more than pitched.
So...
Back in the day (of color film circa 1965) I ran into the problem of reciprocity failure, with reciprocity being the relationship between shutter speed and f-stop. It's the reciprocity that were interested in, not the failure; you can google for that and thank your lucky stars that you're digital these days.
Our cameras are marked in f-stops and shutter speeds, and these speeds have curious increments... 1 sec, 1/2, sec, 1/4 sec, 1/8, sec, and so on. Get it? The one after 1/8 is 1/16, and then 1/32. F stops are the same, but commonly start at 2.0 or so, depending on the lens... So for an 2.0f lens... 2.0, 4.0, 5.6, 8, 11, 16.
Your nikon camera can be set up to increment (or decrement) in 2 or three clicks per f-stop or shutter speed. As an experiment, drag out your camera, set the exposure mode to aperture, and the aperture setting (f-stop) to f8. Notice your shutter speed. Increase your f-stop a full stop (f-11) and your shutter speed will be cut in half. Now go back to f8 and then decrease to f5.6. Your shutter speed will be doubled.
That's it. The relationship between shutter speed and aperture.
So you shot at f16, which would be good for sharpness, and at 1/20 of a sec, which is difficult for sharpness
![Wink ;-)](https://mail.railroadfan.com/phpbb/images/smilies/icon_wink.gif)
(Can't have it all, can we.) 1/40 of a sec would be f11, and 1/60 would be between f11 and f8, but closer to f11. If it were me, I'd set the camera exposure mode on M (for manual) get a light reading at a specific shutter speed (1/60 was recommended, and it sounds good), adjust the f-stop to get the exposure 'zeroed,' snap a test shot, examine the monitor and histogram, and fine tune the f-stop. When the train shows up, snap away!
HTH.
John.