Most of the 1890 census was destroyed by soaking in water, due to a fire in a neighboring building. The records were then left to mold and mildew for many years in a humid Washington D.C. warehouse until they were eventually thrown out. Some fragments were saved, with 6,160 names, but out of the 63 million enumerated that is a very small amount.
Dick Eastman had a small article in his blog on a special article about the 1890 census, written by Tamie Dehler, which appeared in the Terre Haute Tribune Star newspaper. Dehler tells of the Special Enumeration of Union Veterans and Widows, which was requested by the U.S. Pension Office, in order to locate Union Civil War veterans or their widows. Each schedule asked for a name, rank, company, regiment, or vessel, enlistment date, any service connected disability, and general remarks.
Since the enumeration was for the U.S. Pension Office, it was supposed to be only for Union soldiers and their widows because Confederate soldiers were not eligible for a U.S. Pension. Many of the southern states did pay pensions to Confederate soldiers. The enumerators did list non Civil War veterans, and Confederate soldiers, and even some people that never served in any military unit hoping to get an undeserved pension. Widows may have remarried and the schedule lists their new married names and places of enumeration. These schedules also listed Negro Troops for those looking for African American ancestors.
I have used the Washington Union Veterans census, and I was wondering why some of the names had a line drawn through them. Later I learned that they were Confederate soldiers. The use of a single line through the names, left each name, still easy to read. I don't think there were any War of 1812, Mexican War or Indian War veterans listed in the Washington microfilm, but some states did list all veterans, so you might find some non Civil War veterans listings.
The original intent of this enumeration was to make this information a matter of public record, as well as to veterans' organizations, but lack of funding scuttled that idea. In 1930 these schedules were delivered to the newly created Veterans Administration, and in 1943 they were transferred to the National Archives. Not all the enumerations survived and most all of the schedules from the states Alabama through Kansas are missing as well as about half of the Kentucky records. The remainder of Kentucky through Washington, D.C. records survived and are listed in the National Archives Publication M123 and consist of 118 rolls of microfilm. The microfilm also contains some fragments for California, Delaware, Florida, Idaho, Illinois (Cook and Henderson counties), Indiana (Warrick and White counties), and Kansas.