diff --git a/src/unmovable_cmd.cpp b/src/unmovable_cmd.cpp
--- a/src/unmovable_cmd.cpp
+++ b/src/unmovable_cmd.cpp
@@ -291,7 +291,7 @@ static CommandCost ClearTile_Unmovable(T
 	return CommandCost();
 }
 
-static void GetAcceptedCargo_Unmovable(TileIndex tile, AcceptedCargo ac)
+static void AddAcceptedCargo_Unmovable(TileIndex tile, AcceptedCargo ac)
 {
 	if (!IsCompanyHQ(tile)) return;
 
@@ -303,13 +303,13 @@ static void GetAcceptedCargo_Unmovable(T
 
 	/* Top town building generates 10, so to make HQ interesting, the top
 	 * type makes 20. */
-	ac[CT_PASSENGERS] = max(1U, level);
+	ac[CT_PASSENGERS] += max(1U, level);
 
 	/* Top town building generates 4, HQ can make up to 8. The
 	 * proportion passengers:mail is different because such a huge
 	 * commercial building generates unusually high amount of mail
 	 * correspondence per physical visitor. */
-	ac[CT_MAIL] = max(1U, level / 2);
+	ac[CT_MAIL] += max(1U, level / 2);
 }
 
 
@@ -501,7 +501,7 @@ extern const TileTypeProcs _tile_type_un
 	DrawTile_Unmovable,             // draw_tile_proc
 	GetSlopeZ_Unmovable,            // get_slope_z_proc
 	ClearTile_Unmovable,            // clear_tile_proc
-	GetAcceptedCargo_Unmovable,     // get_accepted_cargo_proc
+	AddAcceptedCargo_Unmovable,     // add_accepted_cargo_proc
 	GetTileDesc_Unmovable,          // get_tile_desc_proc
 	GetTileTrackStatus_Unmovable,   // get_tile_track_status_proc
 	ClickTile_Unmovable,            // click_tile_proc