--- a/newgrf_callbacks.h	Tue May 02 21:42:11 2006 +0000
+++ b/newgrf_callbacks.h	Tue May 02 21:43:47 2006 +0000
@@ -25,6 +25,8 @@
 
 	CBID_TRAIN_ARTIC_ENGINE         = 0x16,
 
+	CBID_TRAIN_ALLOW_WAGON_ATTACH   = 0x1D,
+
 	/* This callback is called from vehicle purchase lists. It returns a value to be
 	 * used as a custom string ID in the 0xD000 range. */
 	CBID_VEHICLE_ADDITIONAL_TEXT    = 0x23,

--- a/train_cmd.c	Tue May 02 21:42:11 2006 +0000
+++ b/train_cmd.c	Tue May 02 21:43:47 2006 +0000
@@ -27,6 +27,7 @@
 #include "train.h"
 #include "newgrf_callbacks.h"
 #include "newgrf_engine.h"
+#include "newgrf_text.h"
 #include "direction.h"
 
 static bool TrainCheckIfLineEnds(Vehicle *v);
@@ -1076,6 +1077,17 @@
 		if (flags & DC_EXEC) src->unitnumber = unit_num;
 	}
 
+	if (dst_head != NULL) {
+		/* Check NewGRF Callback 0x1D */
+		uint16 callback = GetVehicleCallbackParent(CBID_TRAIN_ALLOW_WAGON_ATTACH, 0, 0, dst_head->engine_type, src, dst_head);
+		if (callback != CALLBACK_FAILED) {
+			if (callback == 0xFD) return CMD_ERROR;
+			if (callback < 0xFD) {
+				StringID error = GetGRFStringID(GetEngineGRFID(dst_head->engine_type), 0xD000 + callback);
+				return_cmd_error(error);
+			}
+		}
+	}
 
 	/* do it? */
 	if (flags & DC_EXEC) {