The depot appears to be used by Norfolk Southern as office space for MoW crews.
Contributing Property to the Union Downtown Historic District
Per the NRHP Nomination form for the Union Downtown Historic District, it was built ca. 1902.
I have been only able to find out scant information about this depot. So far, normally reliable sources of information about South Carolina depots has turned up very little in the way of a build date and the date passenger service ended here. I will keep looking.