Location
The old Riverside Soldier station was 4 miles from the entrance to Yellowstone National Park.
The 1916 USGS stream gauge description describes the location (at right, link).
Construction
It was built in 1886, and could hold 10 people. (at right, Managing the Matchless Wonders link). "[This building was] constructed or rehabilitated from the former park [...] mail station (Riverside) [...]" (link).
Purpose
Enforce regulations
Prevent illegal harvesting of game (poachers)
In an 1892 account by Georgina Synge, "Here we were accosted by two soldiers in uniform, who asked us if we had any guns to declare, as, if we had, they must be sealed up, to prevent our using them while passing through"
Current Status
The building is no longer present.
"The U.S. Army officially ceded control of the park to the Department of Interior in 1918 (Battle and Thompson 1972)." (link)