Stabilizer on 100-400 lens

If you’re panning to follow a train, ON and Mode 2 are the appropriate settings.

If you’re on a tripod, OFF. If you’re using fast shutter speeds (1/1000 s or faster, conservatively), it won’t help but nor will it hurt to leave it on. Mode 1 is general purpose (holding the lens steady or tracking an erratic subject), Mode 3 is like Mode 1 but it doesn’t start stabilizing until you press the shutter.

One thing to keep in mind with all lens image stabilization systems is that it takes about half a second for stabilization to fully kick in. IS is activated when you half press the shutter (or the AF-ON button if you’re using back button focus), but if you just mash the shutter button down the IS system can actually make the first few resulting images blurry until the system fully engages.

The focus seems to be midway on the train. You might open the raw file up in DPP to verify the focus point. Were you in one shot or servo mode? Did you focus with a half press when the train was farther away and then reframe as it got closer before pressing the full shutter? A look at the raw file parameters might help.

Also, you didn't mention the camera (FF or crop, MP) and whether you cropped or resized the image. Or how much you expect to be in focus. Just the engine or front of the engine? Or the entire train.

Guessing you are standing 100 ft from the train at f/8, the depth of field (DOF) is only about 50 ft on FF, 30 ft on crop. So, probably less than 2 cars should be in focus. It appears car 2 is in best focus, so the engine may be out of the DOF if you prefocused / focus and composed. Closer and the DOF will be shorter - farther and it will be longer. You need to be f/16 or greater to get the entire train in focus from 100 feet on FF at 176mm, or use a shorter focal length to increase the DOF.

If you are a stickler for focus and your body supports it, you might want to make sure the One Shot AF Release Priority is set to Focus Priority and not Release Priority.