49 CFR § 386.40 - Supplementation of responses.
A party who has responded to a request for discovery with a response that was complete when made is under no duty to supplement his/her response to include information thereafter acquired, except as follows:
(a) A party is under a duty to supplement timely his/her response with respect to any question directly addressed to:
(1) The identity and location of persons having knowledge of discoverable matters; and
(2) The identity of each person expected to be called as an expert witness at the hearing, the subject matter on which he or she is expected to testify and the substance of his or her testimony.
(b) A party is under a duty to amend timely a prior response if he or she later obtains information upon the basis of which:
(1) he or she knows the response was incorrect when made; or
(2) he or she knows that the response though correct when made is no longer true and the circumstances are such that a failure to amend the response is in substance a knowing concealment.