Cabooses like box cars need to be waterproof. So it is unlikely they would have relied on paint alone. But their shorter configuration with the raised cupola would have made them less suitable for a standing seam Murphy roof as was used on the box cars.
I believe the caboose roofs could have been Malthoid, an early name for roll roofing with an asphalt or bituminous (coal tar pitch) base. Malthoid and other brand name products were given mineral coatings in order to allow walking on the surface. And the minerals embedded into the sheet could be crushed brick (red), crushed black aggregates (black), or crushed limestone (grey). 1920s and earlier houses had Malthoid roofs which allowed a much lower pitch, even flat. Some were called “Malthoid bungalows”. There is evidence that Malthoid was used in Australia and New Zealand before in the US.
I haven’t researched whether the railroads specifically used Malthoid. But it would be logical. Caboose roofs needed to be walkable. If it was used, the roll material could be nailed at the perimeter and lapped under the roof walk. That way, they would have had no visible seams. I don’t recall any seams in overhead photos.
On Aug 2, 2018, at 8:45 AM, rick@... wrote:James G. Spencer, Architect, AIA