Paid excess RRTA 2 taxes through 2 employers. What do I do with the form 843? Does it go with my 1040 or mailed separately?

I paid a little excess RRTA 2 taxes as a result of having two railroad employers. I have the form 843 and the worksheet but am not sure what to do with it. Does it go with my 1040 and my tax payment? Or does it get mailed separate from my taxes? Thank you.