A US-to-Mexico rail shipment hits four parallel paperwork streams: the standard rail BoL the originating US carrier needs, the Carta Porte 3.1 complement attached to the Mexican CFDI, the Pedimento filed by an Agente Aduanal (or self-cleared under Article 40), and the AES export filing on the US side if value exceeds $2,500. This guide walks through each in the right order with the precise field-level details that trip up first-time cross-border shippers.
The 6 US-MX Commercial Rail Interchanges
Every cross-border US-MX rail car moves through one of six commercial interchanges. Laredo is by far the busiest (~23-24 trains/day).
| US Side / MX Side | State Pair | US Class I | MX Railroad |
|---|---|---|---|
| Calexico / Mexicali | CA / BC | UP | Ferromex (GMXT/FXE) |
| Nogales / Nogales | AZ / SON | UP | Ferromex |
| El Paso / Cd. Juárez | TX / CHIH | UP + BNSF | Ferromex |
| Eagle Pass / Piedras Negras | TX / COAH | UP + BNSF | Ferromex |
| Laredo / Nuevo Laredo | TX / TAMS | CPKC (ex-KCS) | CPKC de México |
| Brownsville / Matamoros | TX / TAMS | UP | CPKC de México |
Laredo and Brownsville are CPKC-controlled (post-Canadian Pacific / Kansas City Southern merger). The remaining four are UP/BNSF + Ferromex (Grupo México Transportes / FXE).
US-Side Rail BoL Add-Ons
For a US-to-MX move, the standard rail BoL adds the following fields:
- TranspInternac = Sí (international transport flag for the Carta Porte)
- EntradaSalidaMerc = Salida (export from US side)
- PaisOrigenDestino = MEX (destination country)
- HTS code (6-digit minimum) per 19 CFR §123.91 on the commodity description line
- AES ITN when shipment value > $2,500 per HTS line (more on this below)
- Interchange selector — pick one of the 6 commercial crossings above
- Mexican consignee details with RFC (12-char persona moral or 13-char persona física); foreign sender RFC for the US shipper is the generic XEXX010101000
The originating US carrier's electronic waybill carries the cross-border indicator. CBP rail manifest data (§123.91) is filed by the carrier; the BoL feeds it.
Carta Porte 3.1 (TipoTransp=04)
Carta Porte 3.1 is the SAT-required transport complement attached to every CFDI moving goods within or into Mexico. Published 17 June 2024, mandatory 17 July 2024. Replaces v3.0. The CFDI carries the fiscal data (invoice or carrier revenue record); the Carta Porte complement carries the transport-specific data (route, equipment, mercancías, weight, distance).
Critical fields for rail (TipoTransp=04):
| Field | Required | Notes |
|---|---|---|
| Version = 3.1 | Yes | Hard-coded |
| IdCCP (UUID starting with C) | Yes | RFC 4122 UUID, first 3 chars start with "C" |
| TranspInternac | Yes | Sí for cross-border |
| EntradaSalidaMerc | Yes (intl) | Entrada (import) or Salida (export) |
| PaisOrigenDestino | Yes (intl) | USA / CAN ISO code |
| ViaEntradaSalida | Yes (intl) | Catalog c_CveTransporte; rail = 04 |
| TotalDistRec | Yes | Decimal km |
| RegimenAduanero | Yes (intl) | IMD (import) / EXD (export) |
| Ubicaciones (≥5 nodes for rail) | Yes | 1 Origen + ≥4 Destino per SAT FAQ |
| PesoBrutoTotal | Yes | kg only |
| FraccionArancelaria | Yes (intl) | 10-digit HTSUS-equivalent |
| TipoDeServicio | Yes | Catalog c_TipoDeServicio (flete normal, expedido, etc.) |
| TipoDeTrafico | Yes | local / interlineal / derechos de paso |
| Carro (1..n) | Yes | TipoCarro, MatriculaCarro, GuiaCarro, ToneladasNetasCarro |
Foreign RFCs: Use generic XEXX010101000 for a US legal entity (foreign sender / consignee) and XAXX010101000 for a US individual or unspecified foreign party. Include the actual US tax ID (EIN) in NumRegIdTrib and ResidenciaFiscal = USA.
Note: Steel Wheel is launching a Carta Porte 3.1 generator as a separate tool in Phase 4. For now, the rail BoL Builder captures the US-side data and exposes the cross-border XML field schema so your customs broker can populate the Mexican CFDI complement directly.
Agente Aduanal vs Article 40 Self-Clearance
Mexican customs clearance has two legal paths under Article 40 of the Ley Aduanera:
Path A: Agente Aduanal (dominant in practice)
An Agente Aduanal is a licensed customs agent who files the Pedimento on behalf of the importer/exporter. The patente (license) is personal, non-transferable, and authorizes the agent to operate at specific aduanas (customs offices). Per the 19-Nov-2025 DOF reform: patente validity is 20 years renewable, 3-year recertification required, and a new Consejo Aduanero governs grants and suspensions.
Per-aduana relevance: when you pick an interchange (Laredo, Eagle Pass, etc.), your Agente Aduanal's patente must include that aduana. Confirm before booking.
Path B: Self-clearance via Representante Legal Aduanero (Article 40)
The current Ley Aduanera Article 40 permits self-clearance for legal entities through an accredited Representante Legal Aduanero (RLA). The RLA must be:
- A Mexican-national individual
- Current with tax obligations
- An employee of the importer/exporter
- Hold proven foreign-trade experience
This is not "mandatory broker" — the brief that the brief that calls Mexico a "broker-mandatory" jurisdiction is outdated. However, the practical reality is that the RLA accreditation has onerous requirements, Agentes maintain the operational relationships at each aduana, and pedimento filing software/access is broker-controlled. Most US→MX rail shippers still use an Agente Aduanal.
Source: Article 40 of the Ley Aduanera, current text. See our customs broker for rail explainer for when self-clearance pencils out.
Pedimento at SAT VUCEM
The Pedimento is the Mexican customs entry document (analogous to US CBP 7501 + 3461 combined). Filed electronically via SAT's VUCEM portal (Ventanilla Única de Comercio Exterior Mexicano).
- If using Agente Aduanal: the agent files on the importer's behalf.
- If self-clearing under Article 40: the importer files via its RLA through VUCEM.
The Pedimento is not the BoL. The BoL is the commercial transport document; the Pedimento is the customs entry filed at clearance. The BoL data (shipper, consignee, commodity, value, weight, Fracción Arancelaria) feeds the Pedimento.
US AES ITN Filing
US exports to Mexico require Electronic Export Information (EEI) filed via AES (Automated Export System) on the AESDirect portal when:
- Value per HTS classification line exceeds $2,500, OR
- The commodity is on the Commerce Control List (CCL) regardless of value
Filed by the US Principal Party in Interest (USPPI) or the Forwarding Agent. AES generates an Internal Transaction Number (ITN) that must appear on the BoL. For self-filers, the AESDirect portal lives at cbp.gov/trade/aes. See our self-filing customs guide for the broker-replacement math.
USMCA Certification Block
For goods qualifying for preferential origin under USMCA (T-MEC), a Certificate of Origin is required. There's no specific form — it can be on the commercial invoice or a separate document. Minimum data elements per USMCA Article 5.2:
- Importer / Exporter / Producer (any can be the certifier)
- Certifier name + address + tax ID + contact
- Description + HS classification (6-digit) of goods
- Origin criterion (A/B/C/D)
- Blanket period (if applicable)
- Authorized signature + date
The free BoL Builder exposes a "Generate USMCA Certification Statement" checkbox when origin and destination are both USMCA parties.
Using the Free BoL Builder for MX Moves
The Rail Bill of Lading Builder exposes a "Cross-Border MX" mode that:
- Converts weights to kg and distances to km
- Renders the 6-interchange dropdown for ViaEntradaSalida selection
- Surfaces the Agente Aduanal panel with per-aduana advisory copy
- Defaults to Agente Aduanal flow with a self-clearance toggle for sophisticated importers
- Surfaces the USMCA cert checkbox
- Surfaces the AES ITN field when origin = US and value > $2,500
- Exposes the Carta Porte 3.1 XML field schema for handoff to your customs broker
For shippers running consistent cross-border lanes, our free rail rate quote tool returns indicative carload estimates that incorporate cross-border accessorials. Open the BoL Builder →
Frequently Asked Questions
What TipoTransp code is used for rail on Carta Porte 3.1?
Rail uses TipoTransp = 04 (Ferroviario). The c_TipoTransp catalog values are: 01 Autotransporte, 02 Marítimo, 03 Aéreo, 04 Ferroviario. Templates and brokers using "02" for rail are filing the wrong transport-type code; 02 is ocean. The rail value has been 04 since Carta Porte 3.1 was published 17 June 2024 and mandatory from 17 July 2024.
Is a Mexican customs broker (Agente Aduanal) mandatory for rail imports?
No. Under Article 40 of the current Ley Aduanera, Mexico permits self-clearance via an accredited Representante Legal Aduanero (RLA). However, the RLA accreditation has onerous requirements, Agentes Aduanales maintain the operational relationships at each aduana, and pedimento filing software/access is broker-controlled. The overwhelming majority of US→MX rail shipments still use an Agente Aduanal in practice, but it is not legally exclusive.
What are the 6 US-MX rail interchange points?
Calexico/Mexicali (UP/Ferromex), Nogales/Nogales (UP/Ferromex), El Paso/Cd. Juárez (UP+BNSF/Ferromex), Eagle Pass/Piedras Negras (UP+BNSF/Ferromex), Laredo/Nuevo Laredo (CPKC), Brownsville/Matamoros (UP/CPKC de México). Laredo is by far the busiest (~23-24 trains/day). Laredo and Brownsville are CPKC-controlled; the other four are UP/BNSF + Ferromex (Grupo México Transportes).
When is the AES ITN required on a US→MX rail BoL?
AES Electronic Export Information must be filed when shipment value exceeds $2,500 per HTS classification line, or when the commodity is on the Commerce Control List regardless of value. Filed via AESDirect by the USPPI or Forwarding Agent. The Internal Transaction Number (ITN) returned by AES must appear on the BoL. The free Steel Wheel BoL Builder exposes the AES ITN field automatically when origin is US and value triggers the threshold.
How many Ubicacion nodes does a rail Carta Porte 3.1 need?
Per the SAT FAQ Ferroviario, rail Carta Porte 3.1 requires a minimum of 5 Ubicacion nodes: 1 Origen plus 4 or more Destino (intermediate destinos + final destino). The intermediate destinos capture interchange points where the car changes hands between carriers. Each Destino node requires DistanciaRecorrida in km.
What's the Pedimento and how does it relate to the BoL?
The Pedimento is the Mexican customs entry document filed via SAT's VUCEM portal — the closest equivalent to a US CBP 7501 + 3461. It is not the BoL. The BoL is the commercial transport document; the Pedimento is the customs filing. The BoL data (shipper, consignee, commodity, value, weight, Fracción Arancelaria) feeds the Pedimento, which is filed by the Agente Aduanal or by the importer's RLA under Article 40 self-clearance.
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