Under the Railroad Retirement Act of 1974, RRB makes independent determinations of railroad workers' claimed T&P disability using the same general criteria that SSA uses to administer its Disability Insurance (DI) program-that is, the worker must have a medically determinable physical or mental impairment that (1) has lasted (or is expected to last) at least 1 year or is expected to result in death and (2) prevents them from engaging in substantial gainful activity, defined as work activity that involves significant physical or mental activities performed for pay or profit.