When searching the Social Security Death Index you can search b y date of birth and date of death as well as where a person lived and applied for their social security number. This is often times where the office that issued the Social Security number was located. You can also search using the residence at time of death which is the address of record but not necessarily where they lived or died and finally where the burial allowance or death benefit was sent.

According to the Social Security Administration a Social Security Number is composed of three parts totally nine digits. The first three digits is called the area number, the second set of two digits is called the group number and the final set of four digits is the serial number.
The Area Number is assigned by the geographical region. Prior to 1972, cards were issued in local Social Security offices around the country and the Area Number represented the state in which the card was issued. This did not necessarily have to be the state where the applicant lived, since a person could apply for their card in any Social Security office. Since 1972, when the Social Security Administration began assigning Social Security Numbers and issuing cards centrally from Baltimore, the area number assigned has been based on the ZIP code in the mailing address provided on the application for the original Social Security card. The applicant's mailing address does not have to be the same as their place of residence. With that said, this number does not necessarily represent the state of residence of the applicant prior to 1972 or currently. However, generally numbers were assigned beginning in the northeast and moving west. People whose cards were issued on the East Coast states have the lowest numbers and those on the West Coast have the highest numbers.

The next two digits of the number are a code used to track fraudulent numbers and the last four digits are randomly assigned.
Although railroad employees were issued Social Security numbers the same as others but from 1937-1963 they had numbers ranging from 700-728 as the first three digits. In 1964 their numbers began to reflect the same as other workers. Some railroad workers received Social Security benefits and some did not so it is wise to check the Social Security Death Index in any case.

If the person you are searching for is a retired railroad worker the US Railroad Retirement Board was created in the 1930’s and has records dating back to 1937 but are for only those whose employers were covered under the Railroad Retirement Act. These records can be obtained for genealogical purposes however there is a fee of $21. The information is filed by Social Security Number. A written request along with a check or money order can be sent to Railroad Retirement Board, 844 North Rush Street, Chicago, IL 60611.