49 C.F.R. § 9.15
(a) Any party seeking the testimony of an employee in a legal proceeding between private litigants, concerning facts within the employee's personal knowledge with regard to matters arising out of the employee's official duties, shall, rather than serving a demand for the testimony, request the testimony at least 30 days before it is intended to be taken or received. The request must be submitted to agency counsel and must include:
(7) A declaration that the party will not seek expert or opinion testimony from the witness or seek the testimony of the witness at a hearing or trial in the proceeding.
The request shall specify which form of testimony (deposition, affidavit, declaration, or answers to interrogatories) is desired and the date by which it is desired; however, only one form, the one least burdensome to the Department that will provide the needed information, will be permitted for each witness.