diff --git a/src/newgrf_cargo.cpp b/src/newgrf_cargo.cpp
--- a/src/newgrf_cargo.cpp
+++ b/src/newgrf_cargo.cpp
@@ -100,14 +100,22 @@ uint16 GetCargoCallback(CallbackID callb
 	return group->GetCallbackResult();
 }
 
-
+/**
+ * Translate a GRF-local cargo slot/bitnum into a CargoID.
+ * @param cargo   GRF-local cargo slot/bitnum.
+ * @param grffile Originating GRF file.
+ * @param usebit  Defines the meaning of \a cargo for GRF version < 7.
+ *                If true, then \a cargo is a bitnum. If false, then \a cargo is a cargoslot.
+ *                For GRF version >= 7 \a cargo is always a translated cargo bit.
+ * @return CargoID or CT_INVALID if the cargo is not available.
+ */
 CargoID GetCargoTranslation(uint8 cargo, const GRFFile *grffile, bool usebit)
 {
 	/* Pre-version 7 uses the 'climate dependent' ID, i.e. cargo is the cargo ID */
 	if (grffile->grf_version < 7) {
 		if (!usebit) return cargo;
 		/* Else the cargo value is a 'climate independent' 'bitnum' */
-		if (HasBit(_cargo_mask, cargo)) return GetCargoIDByBitnum(cargo);
+		return GetCargoIDByBitnum(cargo);
 	} else {
 		/* If the GRF contains a translation table... */
 		if (grffile->cargo_max > 0) {
@@ -116,7 +124,7 @@ CargoID GetCargoTranslation(uint8 cargo,
 			if (cargo < grffile->cargo_max) return GetCargoIDByLabel(grffile->cargo_list[cargo]);
 		} else {
 			/* Else the cargo value is a 'climate independent' 'bitnum' */
-			if (HasBit(_cargo_mask, cargo)) return GetCargoIDByBitnum(cargo);
+			return GetCargoIDByBitnum(cargo);
 		}
 	}
 	return CT_INVALID;