Pub 17

Credit for Excess Social Security Tax or Railroad Retirement Tax Withheld 

   Most employers must withhold social security tax from your wages. Certain
   government employers (some federal, state, and local governments) do not have to
   withhold social security tax. 

   If you work for a railroad employer, that employer must withhold tier 1 railroad
   retirement (RRTA) tax and tier 2 RRTA tax. 

   If you worked for two or more employers in 1997, too much social security tax or
   RRTA may have been withheld from your pay. You can claim the excess as a
   credit against your income tax. The following table shows the maximum amount of
   wages subject to tax and the maximum amount of tax that should have been
   withheld in 1997.