What kind of disability did he receive? It sounds like Social Security disability, which would have been reported to you on an SA-1099 form. Is that the form you get each year reporting that income to you? It sounds like you reported it as railroad retirement instead of social security. some of Social security, even if it is disability, can be taxable, depending on your other income. There is a telephone number on the notice you were sent. Call that number and discuss it with them to make sure you understand what is wrong on your returns. Ask them if you need to send an amended return or if they can correct it for you. Since this sounds like you made a simple entry error, there is nothing we can do for you except tell you to call them. They will not discuss your return with anyone but you. The people in this department at the IRS are usually very pleasant to deal with and very helpful.
