[Excerpt] The Railroad Retirement Act authorizes retirement, survivor, and disability benefits for railroad workers and their families. The Railroad Retirement Board (RRB), an independent federal agency, administers these benefits. Workers covered by the RRB include those employed by railroads engaged in interstate commerce and related subsidiaries, railroad associations, and railroad labor organizations. These benefits are earned by railroad workers and their families in lieu of Social Security. Railroad retirement benefits are divided into two tiers. Tier I benefits are generally computed using the Social Security benefit formula, on the basis of earnings covered by either the Railroad Retirement or Social Security programs. In some cases, RRB Tier I benefits can be higher than comparable Social Security benefits. For example, RRB beneficiaries may receive unreduced Tier I retirement benefits as early as aged 60 if they have at least 30 years of railroad service; Social Security beneficiaries may receive unreduced retirement benefits only when they reach their full retirement ages, currently rising from aged 65 to 67. RRB Tier II benefits are similar to private pension benefits and are based only on railroad work. The Tier I railroad retirement benefit that is equivalent to Social Security benefits is mainly finance by Tier I payroll taxes (typically the same rate as the 12.4% Social Security payroll tax) and Social Security’s financial interchange transfers.3 Tier II benefits, Tier I benefits in excess of Social Security benefits, and supplemental annuities4 are mainly financed by Tier II payroll taxes (currently 13.1% on employers and 4.9% on employees) and transfers from the National Railroad Retirement Investment Trust (NRRIT; hereinafter, the Trust).