49 CFR 1113.15: Interlocutory appeals.

Rulings of the presiding officer may be appealed prior to service of the initial decision only if:

(a) The ruling denies or terminates any person's participation,

(b) The ruling grants a request for the inspection of documents not ordinarily available for public inspection,

(c) The ruling overrules an objection based on privilege, the result of which ruling is to require the presentation of testimony or documents, or

(d) The presiding officer finds that the ruling may result in substantial irreparable harm, substantial detriment to the public interest, or undue prejudice to a party.