A census of Union Civil War soldiers and widows was taken in 1890. The following information was recorded:
name of soldier, sailor, marine or widow - rank - regiment or vessel - date enlisted - date discharged - length of service - PO address - disability incurred - remarks
The following states are indexed on Ancestry.com: D.C., KY, LA, ME, MD, MA, MI, MN, MS, MO, MT, NE, NV, NH, NY, NJ, NM, NC, ND, OH, OK/IND TERR., OR, PA, RI, SC, SD, TN, TX, UT, VT, VA, WA, WV, WI, WY
Check your State Archives or Library for a Veterans' Census if your ancestor lived in the A-D states. For example: Indiana took an Enrollment of Soldiers, Widows and Orphans in 1886, 1890 and 1894. The same basis information is recorded as in the 1890 Vets' enumeration. [Card index at State Library; searchable by county & township at the State Archives]
I located my CW widow, Catherine Crail, in the 1886 Enrollment. Be aware of spelling errors! Catherine was indexed under 'Creal.'