49 CFR §1242.29

Separate common expenses in the running subactivity in the same proportion as the salaries and wages, way and structures, common expenses in all accounts with a designated running subactivity. Separate common expenses in the switching subactivity in the same proportion as the salaries and wages, way and structure, common expense accounts with a designated switching activity. Separate common expenses in the other subactivity in the same proportion as the salaries and wages, way and structures, common expenses in all accounts with a designated other subactivity.