My question is camera related?

My question is camera related. When taking a phot moving subject (Say a Train) I'm setting my camera to Continues Focus and selecting Single point or multi point even Group Focus points and when i do a few test shots there fine yet when the train arives and passes when i review my image after the shot I find the train is not quite in focus were the focus point was set yet the back ground is. Can I say I've tried standing closer/further away I've even tried Pre focusing to no avail. My camera is set to back button focus an I have my camera Sat on a mini bean bag so there's no camera shake one odd thing tho as i hold my back button and fire off shots as the train passes the auto focus point doesn't move which I thought it might. Also I no my camera focus is working because if I hold my camera at some thing and hold the back button and move the camera around it continusly refocuses… Please help my camera is a nikon D500 with a Tamron 70-200 lens No VC or image stabilisation

I don't think you're supposed to be holding the back focus button down while you're shooting. Press it once to establish the subject of the focus.

Sounds like you locked the focus.

Your problem is that you are holding down the AF button as the train passes. You need to set your focusing point and wait for the train to enter that zone, and then press the shutter button. Of course, as the speed of the train increases, you have to press the shutter button earlier and earlier to account for the lag of your reflexes and the camera, too.

Many cameras like the D500 have auto-focus algorithms that look for objects that suddenly appear in the frame. They're programed to ignore such objects. For example, shooting a football game while on the sidelines. Often players and the refs will get in front of the lens. In these situations, you do want the camera to ignore the player or ref and keep focus on the play. This could be, in part, what's happening in your case.

Did you track and follow the subject while taking the picture?

Link to some sample pictures? Your exposure settings? Speed of the train?

Go for manual focus at the approximate distance of the train.