"Railroad employees had a separate retirement program for many years into which they contributed funds, just like Social Security, and received retirement benefits. Participants in this program were a different identification number, one of a block assigned by the Social Security Administration specifically for this program in order to differentiate from standard Social Security participants. Numbers used began with 700-728. This fund is administered by the Railroad Retirement Board. If your ancestor was employed by the railroad, you may want to investigate the availability or records from the Railroad Retirement Board. Consult their Web site at http://www.rrb.gov for more information.
Be sure to also check their Genealogy section at http://www.rrb.gov/mep/genealogy.asp for details of railroad research."