From my point of view, the only reason a brakeman would have been on top of a moving train after the installation of automatic air brakes and prior to the removal and elimination of roof running boards, would have been to: set or knock down the retainers: or relay signals when switching. Having to move to a specific car to check a hand brake or sticking brake may be a possibility as well. There may also be an emergency situation that may have required it, but that would be rare.
I think the photos we have seen in the 30's, 40's, 50's and 60's are employees performing these tasks.
I too observed brakeman in the early days of the C&TS riding the tops of the cars, and even then I could not understand why, as they were not performing those tasks described.
I have walked the tops of (moving) cars to perform these tasks in my younger days, and in many cases I could have waited for the train to stop or done it differently. It is a huge risk, that in my mind, unless absolutely necessary, not worth it.
For all its worth.......
MD Ramsey