It’s slightly longer: 29 railroad miles to Selma vs. 26 to Durham. The train goes pretty fast, though the grade crossings in Clayton and Garner slow it down. I think the bigger factor is that the timetable lists the departure time at Raleigh, so the “Selma” distance includes the long Raleigh station stop.
I think the plan is to keep running the Carolinian through Selma, but once the S-Line opens the commuter rail level frequencies will be headed through Wake Forest, not Clayton.
I’m sure you noticed that Selma is a working-class town; per capita incomes there are less than half the MSA average. Smithfield is a little more prosperous, but both are old mill towns. It looks pretty sad from the train, not nearly as lively as Wilson (but way better than Rocky Mount, ooof).