20 CFR § 222.52 - When a legally adopted child is dependent—general.

(a) During employee's lifetime. If the employee adopts a child before he or she could become entitled to a social security benefit (treating his or her railroad compensation as wages under that Act), the child is considered dependent upon the employee. If the employee adopts a child, unless the child is his natural child or stepchild, after he or she could become entitled to an old age or disability benefit under the Social Security Act (treating his or her railroad compensation as wages under that Act), the child is considered dependent on the employee only if the requirements of § 222.53 are met.

(b) After employee's death. If the surviving spouse of an employee adopted a child after the employee's death, the child is considered dependent on the employee if either—

(1) The employee began proceedings to adopt the child prior to his or her death, or the surviving spouse adopted the child within two years after the employee's death; and

(2) The child was living in the employee's household at the time of the employee's death; and

(3) The child was not receiving regular contributions from any person, including any public or private welfare organization, other than the employee or spouse at the time of the employee's death.