The level crossing barriers are open and therefore the footpath and road are - the area is immediately in front of the signal box and therefore under immediate direct control. As such where they were is safer than standing on a platform surely?
If this was a guard, as has been suggested in other posts, then they would have been at the rear of their train and on leaving Sleights that train would have been heading away from that crossing, on a single line middle of the section with the next potential service a minimum of 40 minutes away - though the timetable means that no trains cross at the next crossing point (Section) Glaisdale. The guard would therefore have been aware that the next service was over 2 hours away ( before anyone says he would have known of any special train movements). So no doubt why he didn't report it, or yell at them to get off the tracks. Why this then became a story is a moot point. I agree with the poster about the location though - how uninspiring unless one of the couple had some connection with the railway/station - maybe that's why the guard took the photo?