James Marshall Plaskitt, 80, who owned a railway equipment and supplies sales firm in Washington, died in his sleep Monday while on a fishing trip in Palatka, Fla. He suffered from a heart ailment.

Mr. Plaskitt established J. M. Plaskitt and Son in 1944 after retiring as chief mechanical officer of Southern Railway. He had joined the railroad in 1924, working for it in Macon, Ga., Charlotte, N.C., Birmingham, Ala., Cincinnati and Washington.

Born in Baltimore, Mr. Plaskitt graduated with the class of 1920 at the Naval Academy in Annapolis. He resigned from the Navy four years later.

He moved to Alexandria in the late 1930's and about 10 years later moved to Upperville, Va., where he maintained Kirkby Farm for feeder cattle.

Mr. Plaskitt was a member of the Metropolitan and Army and Navy clubs. He was a former senior warden and treasurer of Trinity Episcopal Church in Upperville.

He is survived by a son, John Marshall Jr., of Upperville; three daughters, Mary Welby Brown, of Milwood, Va., Courtenay Morris Hanson, of Alexandria, and Virginia Ashby McCarty, of Delaplane, Va., and nine grandchildren.

The family suggests that expressions of sympathy may be in the form of contributions to the Book of Remembrance of Trinity Episcopal Church.