# Train: REM methodology v2.4 — docs & API

**What:** The **REM** (Rail Emission Model) train methodology is documented as the primary rail integration path. **`methodology: "REM"`** replaces **`"SQUAKE"`** in examples (legacy alias still accepted). Integration docs and OpenAPI train schemas now include **`departure_date`** (ISO `YYYY-MM-DD`), **`origin_country`** / **`destination_country`** (ISO 3166-1 alpha-2), **`energy_scope`** (`wtw` / `ttw`), optional **`route_coords`** for border-aware distance, and the **tiered factor resolver** (Tier 1 → Tier 3 → Tier 4 → Tier 5; Tier 2 is not used in production).

**Do now:** Use [Train: REM](/api-calculations-request-response/travel/train/train-rem.md) for field definitions and request shapes; prefer **`REM`** in new integrations.

**Reference:** REM factor bundle and loader notes are maintained alongside the calculation service; methodology narrative follows REM v2.4.


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