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 19:52:29 2011 +0000
@@ -70,7 +70,7 @@
    /** 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. */
+   /** Determine whether an already existing consist will allow a wagon to be attached. */
    CBID_TRAIN_ALLOW_WAGON_ATTACH        = 0x1D,

    /** Called to determine the colour of a town building. */
@@ -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 19:52:29 2011 +0000
@@ -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 19:52:29 2011 +0000
@@ -971,6 +971,7 @@
            t->InvalidateNewGRFCache();

            uint16 callback = GetVehicleCallbackParent(CBID_TRAIN_ALLOW_WAGON_ATTACH, 0, 0, head->engine_type, t, head);
+           uint16 callback2 = GetVehicleCallback(CBID_WAGON_ALLOW_ATTACH_TO_CONSIST, 0, 0, t->engine_type, t);

            /* Restore original first_engine data */
            t->gcache.first_engine = first_engine;
@@ -988,6 +989,17 @@

                if (error != STR_NULL) return_cmd_error(error);
            }
+
+           if (callback2 != 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(t->engine_type), 0xD000 + callback);
+
+               if (error != STR_NULL) return_cmd_error(error);
+           }
+
        }

        /* And link it to the new part. */