In his article, "Social Security Death Index (SSDI) Overview," Alan Smith provides a basic overview and insights into the Social Security Death Index (SSDI). The article notes that the SSDI includes some 400,000 railroad retirement records from early 1900s to 1950s. To clarify, we have this from the RootsWeb site quoted in the article, "Railroad workers were enrolled in the same Social Security program, but from 1937 to 1963 they had numbers ranging between 700 and 728 as the first three digits. In 1964 their numbers began to reflect the same geographic location as other workers. Some railroad workers received Social Security benefits, but some did not. However, it is wise to check the SSDI in any case."