Do I need a BOC-3 for a double-trailer operation?
Yes — if the double-trailer operation is interstate and the carrier holds FMCSA operating authority, the BOC-3 requirement applies. The number of trailers per power unit does not affect the §366 process-agent designation; the determining factor is whether the carrier crosses state lines under FMCSA authority.
The 49 CFR §366 process-agent designation rule applies to any "motor carrier" or "broker" subject to FMCSA jurisdiction under §13501. The BOC-3 trigger is interstate operating authority, full stop. The configuration of the equipment — single trailer, double trailer, triple trailer (where state-permitted), tanker, flatbed, hazmat — does not change the §366 obligation.
Doubles permitting is a separate set of state-by-state rules. Some states allow Rocky Mountain Doubles or Turnpike Doubles only on specific corridors, others restrict double-trailer operations to certain weight classes or driver-experience requirements. Those state permits are filed under each state's motor-carrier permitting system, not under federal BOC-3.
For a carrier scaling from singles to doubles, the BOC-3 already on file remains valid as long as the carrier's legal name, USDOT, and MC number do not change. Adding doubles to the operation is an MCS-150 update (vehicle-count change, possibly a new operation-type code) but not a BOC-3 update. The §366 process-agent designation is unaffected by equipment changes.
Carriers running specialized configurations (oversize, hazmat, doubles, agricultural) often layer multiple state-level permits on top of the federal BOC-3. The federal designation stays as a single one-time filing; the state permits are renewed on each state's independent cycle (typically annual).