Went down to Martinsville a couple weekends ago to check out the line. Stopped by the suspected depot and chatted with the owner. She confirmed that the building was the depot and gave a little description of the building layout.
One interesting thing is that the side doorway (the one to the alley) was covered up when the building was constructed on the south side. She owned the main depot building, and bought the south building later. She was elated to find that the door was still there, as that meant she could connect the two buildings internally. She thought the windows also were there, but no need to open them up.
The depot's front door looks to be in the original location, as are the front windows, though they were greatly enlarged. The substation building in the rear is also still there, and the upstairs is open for browsing.
I've attached a couple photos of the depot building. I've also attached an animated image showing the progression between the postcard and today (not quite a perfect then-now shot, but close enough).
Additionally, between Mooresville and Martinsville, the roads that closely parallel the THI&E route have been signed as the "Johnny Wooden Interurban Parkway". This was a tribute to John Wooden's rides on the interurban from his home in Centerton to the basketball gym in Martinsville. It was put up last year I think. I've included one photo of the sign at Main and College on the way out of Martinsville; there are probably two dozen more along the route.