diff -r 50c89d44e35c src/depot_gui.cpp
--- a/src/depot_gui.cpp	Fri Mar 19 13:54:58 2010 +0100
+++ b/src/depot_gui.cpp	Fri Mar 19 13:55:01 2010 +0100
@@ -212,6 +212,7 @@
 	bool generate_list;
 	VehicleList vehicle_list;
 	VehicleList wagon_list;
+	VehicleID vehicle_over;
 
 	DepotWindow(const WindowDesc *desc, TileIndex tile, VehicleType type) : Window()
 	{
@@ -220,6 +221,7 @@
 		this->sel = INVALID_VEHICLE;
 		this->generate_list = true;
 		this->type = type;
+		this->vehicle_over = INVALID_VEHICLE;
 
 		this->CreateNestedTree(desc);
 		this->SetupWidgetData(type);
@@ -256,7 +258,7 @@
 				free_wagon = u->IsFreeWagon();
 
 				uint x_space = free_wagon ? TRAININFO_DEFAULT_VEHICLE_WIDTH : 0;
-				DrawTrainImage(u, image_left + (rtl ? 0 : x_space), image_right - (rtl ? x_space : 0), sprite_y - 1, this->sel, free_wagon ? 0 : this->hscroll.GetPosition());
+				DrawTrainImage(u, image_left + (rtl ? 0 : x_space), image_right - (rtl ? x_space : 0), sprite_y - 1, this->sel, free_wagon ? 0 : this->hscroll.GetPosition(), this->vehicle_over);
 
 				/* Number of wagons relative to a standard length wagon (rounded up) */
 				SetDParam(0, (u->tcache.cached_total_length + 7) / 8);
@@ -855,6 +857,7 @@
 
 		/* abort drag & drop */
 		this->sel = INVALID_VEHICLE;
+		this->vehicle_over = INVALID_VEHICLE;
 		this->SetWidgetDirty(DEPOT_WIDGET_MATRIX);
 	};
 
@@ -870,6 +873,36 @@
 		}
 	}
 
+	virtual void OnMouseDrag(Point pt, int widget)
+	{
+		if (this->type == VEH_TRAIN && this->sel != INVALID_VEHICLE) {
+			/* We are dragging a rail vehicle. */
+
+			if (widget != DEPOT_WIDGET_MATRIX) return;
+
+			NWidgetBase *nwi = this->GetWidget<NWidgetBase>(widget);
+			const Vehicle *v = NULL;
+			GetDepotVehiclePtData gdvp = { NULL, NULL };
+
+			if (this->GetVehicleFromDepotWndPt(pt.x - nwi->pos_x, pt.y - nwi->pos_y, &v, &gdvp) != MODE_DRAG_VEHICLE) return;
+
+			VehicleID new_vehicle_over = INVALID_VEHICLE;
+
+			if (gdvp.head != NULL && gdvp.wagon != NULL && 
+					gdvp.head != gdvp.wagon && 
+					gdvp.wagon->index != this->sel && 
+					gdvp.wagon->Previous()->index != this->sel) {
+				/* Drag over an existing wagon */
+				new_vehicle_over = gdvp.wagon->index;
+			}
+
+			if (this->vehicle_over == new_vehicle_over) return;
+
+			this->vehicle_over = new_vehicle_over;
+			this->SetWidgetDirty(DEPOT_WIDGET_MATRIX);
+		}
+	}
+
 	virtual void OnDragDrop(Point pt, int widget)
 	{
 		switch (widget) {
@@ -889,6 +922,7 @@
 							DoCommandP(Vehicle::Get(sel)->tile, Vehicle::Get(sel)->index, true,
 									CMD_REVERSE_TRAIN_DIRECTION | CMD_MSG(STR_ERROR_CAN_T_REVERSE_DIRECTION_RAIL_VEHICLE));
 						} else if (gdvp.wagon == NULL || gdvp.wagon->index != sel) {
+							this->vehicle_over = INVALID_VEHICLE;
 							TrainDepotMoveVehicle(gdvp.wagon, sel, gdvp.head);
 						} else if (gdvp.head != NULL && Train::From(gdvp.head)->IsFrontEngine()) {
 							ShowVehicleViewWindow(gdvp.head);
diff -r 50c89d44e35c src/train_gui.cpp
--- a/src/train_gui.cpp	Fri Mar 19 13:54:58 2010 +0100
+++ b/src/train_gui.cpp	Fri Mar 19 13:55:01 2010 +0100
@@ -56,8 +56,9 @@
  * @param y         Vertical position to draw at
  * @param selection Selected vehicle to draw a frame around
  * @param skip      Number of pixels to skip at the front (for scrolling)
+ * @param drag_dest The vehicle we are currently dragging another one over
  */
-void DrawTrainImage(const Train *v, int left, int right, int y, VehicleID selection, int skip)
+void DrawTrainImage(const Train *v, int left, int right, int y, VehicleID selection, int skip, VehicleID drag_dest)
 {
 	bool rtl = _dynlang.text_dir == TD_RTL;
 	Direction dir = rtl ? DIR_E : DIR_W;
@@ -79,6 +80,11 @@
 		Point offset;
 		int width = Train::From(v)->GetDisplayImageWidth(&offset);
 
+		if (drag_dest != INVALID_VEHICLE && drag_dest == v->index) {
+			/* Highlight the drag & drop destination inside train: insert a 5px wide space. */
+			px += rtl ? -5 : 5;
+		}
+
 		if (rtl ? px + width > 0 : px - width < max_width) {
 			PaletteID pal = (v->vehstatus & VS_CRASHED) ? PALETTE_CRASH : GetVehiclePalette(v);
 			DrawSprite(v->GetImage(dir), pal, px + (rtl ? -offset.x : offset.x), 7 + offset.y);
diff -r 50c89d44e35c src/vehicle_gui.h
--- a/src/vehicle_gui.h	Fri Mar 19 13:54:58 2010 +0100
+++ b/src/vehicle_gui.h	Fri Mar 19 13:55:01 2010 +0100
@@ -64,7 +64,7 @@
 
 int DrawVehiclePurchaseInfo(int left, int right, int y, EngineID engine_number);
 
-void DrawTrainImage(const Train *v, int left, int right, int y, VehicleID selection, int skip);
+void DrawTrainImage(const Train *v, int left, int right, int y, VehicleID selection, int skip, VehicleID drag_destination = INVALID_VEHICLE);
 void DrawRoadVehImage(const Vehicle *v, int left, int right, int y, VehicleID selection);
 void DrawShipImage(const Vehicle *v, int left, int right, int y, VehicleID selection);
 void DrawAircraftImage(const Vehicle *v, int left, int right, int y, VehicleID selection);