In the case of a railroad line which carried less than 3,000,000 gross ton miles of traffic per mile in the preceding calendar year, whenever a purchasing carrier under this section petitions the Board for joint rates applicable to traffic moving over through routes in which the purchasing carrier may practicably participate, the Board shall, within 30 days after the date such petition is filed and pursuant to
section 10705(a) of this title, require the establishment of reasonable joint rates and divisions over such route.