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, your employer must withhold Tier 1 railroad retirement (RRTA) tax and Tier 2 RRTA tax.
If you had more than one employer and your total wages were over the wage base limit for the year, too much social security tax or Tier 1 RRTA may have been withheld. The wage base limit for the year can be found in the Form 1040 Instructions. If you had more than one railroad employer, and your total compensation was over the maximum amount of wages subject to Tier 2 RRTA, too much Tier 2 railroad retirement (RRTA) tax may have been withheld. If you had too much social security tax or Tier 1 RRTA withheld, you may be able to claim the excess as a credit against your income tax. If any one employer withheld too much social security or RRTA tax, you cannot claim the excess as a credit against your income tax. Your employer should make an adjustment of the excess for you. If the employer does not make an adjustment, you can use Form 843 (PDF), Claim for Refund and Request for Abatement to claim a refund.
For details, including how to compute the amount of excess credit, refer to Publication 505, Tax Withholding and Estimated Tax.
Also, please note, OnLine Taxes does not support Form 843.