diff --git a/newgrf_callbacks.h b/newgrf_callbacks.h
--- a/newgrf_callbacks.h
+++ b/newgrf_callbacks.h
@@ -19,6 +19,10 @@ enum CallbackID {
 	// only for train vehicles
 	CBID_TRAIN_VEHICLE_LENGTH       = 0x11,
 
+	/* Called to determine the amount of cargo to load per unit of time when
+	 * using gradual loading. */
+	CBID_VEHICLE_LOAD_AMOUNT        = 0x12,
+
 	/* Called (if appropriate bit in callback mask is set) to determine if a
 	 * newstation should be made available to build */
 	CBID_STATION_AVAILABILITY       = 0x13,