20 CFR 211.11: Miscellaneous pay.

Any payment made to an employee by an employer which is excluded from compensation under the Railroad Retirement Act, but which is subject to taxes under the Railroad Retirement Tax Act, shall be considered compensation for purposes of this part but only for the limited purpose of computing the portion of the annuity computed under section 3(a), 4(a), or 4(f) of the Railroad Retirement Act (commonly called the tier I component).