diff -r a33e0abe08ce src/newgrf_callbacks.h
--- a/src/newgrf_callbacks.h	Wed Jan 12 15:33:47 2011 +0000
+++ b/src/newgrf_callbacks.h	Sat Jan 22 21:42:23 2011 +0000
@@ -70,8 +70,8 @@
 	/** Called whenever the construction state of a house changes. */
 	CBID_HOUSE_CONSTRUCTION_STATE_CHANGE = 0x1C, // 15 bit callback
 
-	/** Determine whether a wagon can be attached to an already existing train. */
-	CBID_TRAIN_ALLOW_WAGON_ATTACH        = 0x1D,
+	/** Determine whether the lead engine of a consist will allow a wagon to be attached. */
+	CBID_ENGINE_ALLOW_WAGON_ATTACH       = 0x1D,
 
 	/** Called to determine the colour of a town building. */
 	CBID_HOUSE_COLOUR                    = 0x1E, // 15 bit callback
@@ -272,6 +272,10 @@
 
 	/** Called to determine if one can alter the ground below an object tile */
 	CBID_OBJECT_AUTOSLOPE                = 0x15D, // 15 bit callback
+	
+	/** Determine whether a wagon will allow attaching to an existing consist. */
+	CBID_WAGON_ALLOW_ATTACH_TO_CONSIST   = 0x15E,
+	
 };
 
 /**
diff -r a33e0abe08ce src/table/newgrf_debug_data.h
--- a/src/table/newgrf_debug_data.h	Wed Jan 12 15:33:47 2011 +0000
+++ b/src/table/newgrf_debug_data.h	Sat Jan 22 21:42:23 2011 +0000
@@ -32,7 +32,7 @@
 	NICV(CBID_VEHICLE_REFIT_CAPACITY,        CBM_VEHICLE_REFIT_CAPACITY),
 	NICV(CBID_VEHICLE_ARTIC_ENGINE,          CBM_VEHICLE_ARTIC_ENGINE),
 	NICV(CBID_VEHICLE_CARGO_SUFFIX,          CBM_VEHICLE_CARGO_SUFFIX),
-	NICV(CBID_TRAIN_ALLOW_WAGON_ATTACH,      CBM_NO_BIT),
+	NICV(CBID_ENGINE_ALLOW_WAGON_ATTACH,     CBM_NO_BIT),
 	NICV(CBID_VEHICLE_ADDITIONAL_TEXT,       CBM_NO_BIT),
 	NICV(CBID_VEHICLE_COLOUR_MAPPING,        CBM_VEHICLE_COLOUR_REMAP),
 	NICV(CBID_VEHICLE_START_STOP_CHECK,      CBM_NO_BIT),
@@ -40,6 +40,7 @@
 	NICV(CBID_VEHICLE_SOUND_EFFECT,          CBM_VEHICLE_SOUND_EFFECT),
 	NICV(CBID_VEHICLE_AUTOREPLACE_SELECTION, CBM_NO_BIT),
 	NICV(CBID_VEHICLE_MODIFY_PROPERTY,       CBM_NO_BIT),
+	NICV(CBID_WAGON_ALLOW_ATTACH_TO_CONSIST, CBM_NO_BIT),
 	NIC_END()
 };
 
diff -r a33e0abe08ce src/train_cmd.cpp
--- a/src/train_cmd.cpp	Wed Jan 12 15:33:47 2011 +0000
+++ b/src/train_cmd.cpp	Sat Jan 22 21:42:23 2011 +0000
@@ -970,8 +970,9 @@
 			 * the loop and after each callback does not need to be cleared here. */
 			t->InvalidateNewGRFCache();
 
-			uint16 callback = GetVehicleCallbackParent(CBID_TRAIN_ALLOW_WAGON_ATTACH, 0, 0, head->engine_type, t, head);
-
+			uint16 engine_callback = GetVehicleCallbackParent(CBID_TRAIN_ALLOW_WAGON_ATTACH, 0, 0, head->engine_type, t, head);
+			uint16 wagon_callback = GetVehicleCallbackParent(CBID_WAGON_ALLOW_ATTACH_TO_CONSIST, 0, 0, t->engine_type, t, head);
+			
 			/* Restore original first_engine data */
 			t->gcache.first_engine = first_engine;
 
@@ -979,15 +980,26 @@
 			t->InvalidateNewGRFCache();
 			head->InvalidateNewGRFCache();
 
-			if (callback != CALLBACK_FAILED) {
+			if (engine_callback != CALLBACK_FAILED) {
 				/* A failing callback means everything is okay */
 				StringID error = STR_NULL;
 
-				if (callback == 0xFD) error = STR_ERROR_INCOMPATIBLE_RAIL_TYPES;
-				if (callback  < 0xFD) error = GetGRFStringID(GetEngineGRFID(head->engine_type), 0xD000 + callback);
+				if (engine_callback == 0xFD) error = STR_ERROR_INCOMPATIBLE_RAIL_TYPES;
+				if (engine_callback  < 0xFD) error = GetGRFStringID(GetEngineGRFID(head->engine_type), 0xD000 + engine_callback);
 
 				if (error != STR_NULL) return_cmd_error(error);
 			}
+			
+			if (wagon_callback != CALLBACK_FAILED) {
+				/* A failing callback means everything is okay */
+				StringID error = STR_NULL;
+				
+				if (wagon_callback == 0xFD) error = STR_ERROR_INCOMPATIBLE_RAIL_TYPES;
+				if (wagon_callback  < 0xFD) error = GetGRFStringID(GetEngineGRFID(t->engine_type), 0xD000 + wagon_callback);
+				
+				if (error != STR_NULL) return_cmd_error(error);
+			}
+			
 		}
 
 		/* And link it to the new part. */