Of course we can't see what's on top. From the narrative:
Ancient coins indicate preliminary plans to top the column with a statue of a bird, probably an eagle, but after construction, a statue of Trajan was put in place; this statue disappeared in the Middle Ages. On December 4, 1587, the top was crowned by Pope Sixtus V with a bronze figure of St. Peter, which remains to this day.
As far as ancient literary sources are concerned, an extant continuous account of Trajan's reign does not exist.