49 CFR § 105.45 - Issuing a subpoena.
(a) Subpoenas explained. A subpoena is a document that may require you to attend a proceeding, produce documents or other physical evidence in your possession or control, or both. PHMSA may issue a subpoena either on its initiative or at the request of someone participating in a proceeding. Anyone who requests that PHMSA issue a subpoena must show that the subpoena seeks information that will materially advance the proceeding.
(b) Attendance and mileage expenses.
(1) If you receive a subpoena to attend a proceeding under this part, you may receive money to cover attendance and mileage expenses. The attendance and mileage fees will be the same as those paid to a witness in a proceeding in the district courts of the United States.
(2) If PHMSA issues a subpoena to you based upon a request, the requester must serve a copy of the original subpoena on you, as required in § 105.50. The requester must also include attendance and mileage fees with the subpoena unless the requester asks PHMSA to pay the attendance and mileage fees because of demonstrated financial hardship and PHMSA agrees to do so.
(3) If PHMSA issues a subpoena at the request of an officer or agency of the Federal government, the officer or agency is not required to include attendance and mileage fees when serving the subpoena. The officer or agency must pay the fees before you leave the hearing at which you testify.