--- a/src/aircraft.h	Wed May 02 09:29:41 2007 +0000
+++ b/src/aircraft.h	Wed May 02 09:39:11 2007 +0000
@@ -130,11 +130,11 @@
 	/** We want to 'destruct' the right class. */
 	virtual ~Aircraft() {}
 
-	const char *GetTypeString() { return "aircraft"; }
+	const char *GetTypeString() const { return "aircraft"; }
 	void MarkDirty();
 	void UpdateDeltaXY(Direction direction);
-	ExpensesType GetExpenseType(bool income) { return income ? EXPENSES_AIRCRAFT_INC : EXPENSES_AIRCRAFT_RUN; }
-	WindowClass GetVehicleListWindowClass() { return WC_AIRCRAFT_LIST; }
+	ExpensesType GetExpenseType(bool income) const { return income ? EXPENSES_AIRCRAFT_INC : EXPENSES_AIRCRAFT_RUN; }
+	WindowClass GetVehicleListWindowClass() const { return WC_AIRCRAFT_LIST; }
 };
 
 #endif /* AIRCRAFT_H */

--- a/src/roadveh.h	Wed May 02 09:29:41 2007 +0000
+++ b/src/roadveh.h	Wed May 02 09:39:11 2007 +0000
@@ -38,11 +38,11 @@
 	/** We want to 'destruct' the right class. */
 	virtual ~RoadVehicle() {}
 
-	const char *GetTypeString() { return "road vehicle"; }
+	const char *GetTypeString() const { return "road vehicle"; }
 	void MarkDirty();
 	void UpdateDeltaXY(Direction direction);
-	ExpensesType GetExpenseType(bool income) { return income ? EXPENSES_ROADVEH_INC : EXPENSES_ROADVEH_RUN; }
-	WindowClass GetVehicleListWindowClass() { return WC_ROADVEH_LIST; }
+	ExpensesType GetExpenseType(bool income) const { return income ? EXPENSES_ROADVEH_INC : EXPENSES_ROADVEH_RUN; }
+	WindowClass GetVehicleListWindowClass() const { return WC_ROADVEH_LIST; }
 };
 
 #endif /* ROADVEH_H */

--- a/src/ship.h	Wed May 02 09:29:41 2007 +0000
+++ b/src/ship.h	Wed May 02 09:39:11 2007 +0000
@@ -39,11 +39,11 @@
 	/** We want to 'destruct' the right class. */
 	virtual ~Ship() {}
 
-	const char *GetTypeString() { return "ship"; }
+	const char *GetTypeString() const { return "ship"; }
 	void MarkDirty();
 	void UpdateDeltaXY(Direction direction);
-	ExpensesType GetExpenseType(bool income) { return income ? EXPENSES_SHIP_INC : EXPENSES_SHIP_RUN; }
-	WindowClass GetVehicleListWindowClass() { return WC_SHIPS_LIST; }
+	ExpensesType GetExpenseType(bool income) const { return income ? EXPENSES_SHIP_INC : EXPENSES_SHIP_RUN; }
+	WindowClass GetVehicleListWindowClass() const { return WC_SHIPS_LIST; }
 };
 
 #endif /* SHIP_H */

--- a/src/train.h	Wed May 02 09:29:41 2007 +0000
+++ b/src/train.h	Wed May 02 09:39:11 2007 +0000
@@ -243,11 +243,11 @@
 	/** We want to 'destruct' the right class. */
 	virtual ~Train() {}
 
-	const char *GetTypeString() { return "train"; }
+	const char *GetTypeString() const { return "train"; }
 	void MarkDirty();
 	void UpdateDeltaXY(Direction direction);
-	ExpensesType GetExpenseType(bool income) { return income ? EXPENSES_TRAIN_INC : EXPENSES_TRAIN_RUN; }
-	WindowClass GetVehicleListWindowClass() { return WC_TRAINS_LIST; }
+	ExpensesType GetExpenseType(bool income) const { return income ? EXPENSES_TRAIN_INC : EXPENSES_TRAIN_RUN; }
+	WindowClass GetVehicleListWindowClass() const { return WC_TRAINS_LIST; }
 };
 
 #endif /* TRAIN_H */

--- a/src/vehicle.h	Wed May 02 09:29:41 2007 +0000
+++ b/src/vehicle.h	Wed May 02 09:39:11 2007 +0000
@@ -352,7 +352,7 @@
 	 * Get a string 'representation' of the vehicle type.
 	 * @return the string representation.
 	 */
-	virtual const char* GetTypeString() = 0;
+	virtual const char* GetTypeString() const = 0;
 
 	/**
 	 * Marks the vehicles to be redrawn and updates cached variables
@@ -370,12 +370,12 @@
 	 * Sets the expense type associated to this vehicle type
 	 * @param income whether this is income or (running) expenses of the vehicle
 	 */
-	virtual ExpensesType GetExpenseType(bool income) { return EXPENSES_OTHER; }
+	virtual ExpensesType GetExpenseType(bool income) const { return EXPENSES_OTHER; }
 
 	/**
 	 * Invalidates the vehicle list window of this type of vehicle
 	 */
-	virtual WindowClass GetVehicleListWindowClass() { return WC_NONE; }
+	virtual WindowClass GetVehicleListWindowClass() const { return WC_NONE; }
 };
 
 /**
@@ -400,7 +400,7 @@
 	/** We want to 'destruct' the right class. */
 	virtual ~SpecialVehicle() {}
 
-	const char *GetTypeString() { return "special vehicle"; }
+	const char *GetTypeString() const { return "special vehicle"; }
 	void UpdateDeltaXY(Direction direction);
 };
 
@@ -419,7 +419,7 @@
 	/** We want to 'destruct' the right class. */
 	virtual ~DisasterVehicle() {}
 
-	const char *GetTypeString() { return "disaster vehicle"; }
+	const char *GetTypeString() const { return "disaster vehicle"; }
 	void UpdateDeltaXY(Direction direction);
 };
 
@@ -438,7 +438,7 @@
 	/** We want to 'destruct' the right class. */
 	virtual ~InvalidVehicle() {}
 
-	const char *GetTypeString() { return "invalid vehicle"; }
+	const char *GetTypeString() const { return "invalid vehicle"; }
 };
 
 #define is_custom_sprite(x) (x >= 0xFD)