20 C.F.R. § 345.101
Every employer, as defined in part 301 of this chapter, shall pay to the Railroad Retirement Board a contribution with respect to the compensation paid to an employee in any calendar month for service by such employee (except for service to a local lodge or division of a railway labor organization). For the purposes of this part, the term "compensation" is defined in part 302 of this chapter. The compensation subject to contribution is the gross amount of compensation paid to an employee for service in any month, not to exceed the amount of the monthly compensation base (MCB), as defined in part 302 of this chapter. The amount of contribution payable by each employer is to be computed and paid pursuant to the provisions of this part.
20 C.F.R. § 345.101