They do appear to be wearing caps based on the Model 1895 U.S. Army Cap. The use of Russian Shoulder Knots is interesting. One man seems to be wearing a chevron with the point up. Upward pointed chevrons were not used by the U.S. Army during this time frame. Did conductors wear rank chevrons like this? Also 6 button flock coats were not common in the U.S. Army or the U.S. militia. Has anyone thought these could be soldiers ca. 1890s, but not U.S. soldiers?
Also Russian Shoulder Knots were for U.S. Civil War era artillery and these men do not have anything that makes me think artillery or even military other than the sleeve cuff chevrons and upper arm rank chevron.
Shoulder knots would/could also be worn by ACW Marine Corps officers; as would the up-turned chevrons (for enlisted men of course). However I´m not sure if I see the chevrons correctly - the standing person seem to have just a single chevron with a tie, which would most likely make it a non-regulation insignia for a quartermaster sergeant on the company level. Not sure if marines had company quartermaster sergeants.