First damage control:
The Johnson-Cutshall Family tree correct link:http://trees.ancestry.com/tree/21704170/person/1478682823?ss...
You can access it if you have a membership to Ancestry.com; you can also try your local library (using their computer system and your library card, IF they have the library version).
--This tree identifies the father's obit and Thomas being identified as "Emmet Ray", who lived at Fort DuChene, UT.
--It identified Death of Thomas 20 Mar 1915 at Fort Leavenworth, KS, but "no evidence for the burial here being the same as Thomas Ray. Based on comments here in this posting and this family tree, I would "guess" you were the owner.
---Additional information: The 1870 Census of Yell, Boone County, Iowa, identified Thomas E. Ray as being 2/12ths in age. The census was enumerated on 2 August, so if correct, he was born about May/June 1870. Note: This is NOT backed by ANY documentation.
Now, let me answer your actual question:
His being at Fort Duchesne does not prove he was IN the military, but probable. There would be no information in the Utah archives about US Army members assigned, unless one individual had somehow been involved in something under the state jurisdiction or newsworthy.
Just for some TOTAL SPECULATION on this issue of his burial at Fort Leavenworth:
First, if he was alive and identified as being a military member at Fort Leavenworth at time of death. IF Thomas Ray WAS in the military at Fort DuChene in 1898, and he was the correct person that died at Fort Leavenworth, KS in 1915 as a private, there might have been military court martial for a major crime. A military member in 1898 should have progressed well past the rank of private in 17 years. To not so progress, the person might have been found guilty of a major crime, being reduced in rank to private, losing all pay and privileges and be incarcerated at the federal prison at Fort Leavenworth. This person that died in 1915, if the correct Thomas Ray would have been about 45. To me it sounds like two different persons, but the scenario can't be disproven or proven without facts.
---BUT, the actual marker http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GSln=ra...
appears to be a marker reserved for a service member/honorably served. Or, in the possibility in this case, the marker was in regard to his service at ANY point in time previous. The marker shows very little "3403-A Thos. E. Ray, U.S.A. (US Army). The traditional marker for military service is more detailed than what is shown for what could be Spanish American War or WWI service. The same shield was used for Spanish American War veterans, but the unit information was more informative.
--The 3403-A is nothing more than the plot number. The actual transcription from:http://www.interment.net/data/us/ks/leavenworth/fortleavnat/...
"Ray, Thomas E, d. 03/20/1915, PVT 2ND CO, US DISC BARRACKS GUARD, Plot: H 3403-A, *" (this stood for the 2nd company, US Disciplinary Barracks Guard, which to me "indicates" possible service in this capacity at the Fort Leavenworth prison. This also suggests to me [MY SPECULATION] this might have indicated an active duty death while performing duties, possibly an honor stone over a cenotaph (empty grave).
Records with an asterisk at the end indicates those that have not been verified as accurate by the U.S. Department of Veterans' Affairs. Please visit "The National Cemetery Administration Records Verification Project " http://www.interment.net/column/records/nca/index.htm
for details on what this means. To report an error, please visit the Veterans Affairs website, click on "Contact the VA" and follow the instructions.http://www.va.gov/
--I would suggest contact of the cemetery or the Department of Veteran Affairs for information about this person. The VA takes the application for these memorials from family members.
Now, as to any military records. Records would not be at Fort Duchesne, as any service records follow the member, and the service. For pre-WWI records:http://www.archives.gov/veterans/military-service-records/pr...
...BUT, depending on when he served, the 1973 fire may have destroyed the primary records:http://www.archives.gov/st-louis/military-personnel/fire-197...