Add freight train load/unload mode: through load

src/saveload/extended_ver_sl.h

@@ -55,6 +55,7 @@ enum SlXvFeatureIndex {
 	XSLFI_MULTIPLE_DOCKS,                         ///< Multiple docks
 	XSLFI_TIMETABLE_EXTRA,                        ///< Vehicle timetable extra fields
 	XSLFI_TRAIN_FLAGS_EXTRA,                      ///< Train flags field extra size
+	XSLFI_TRAIN_THROUGH_LOAD,                     ///< Train through load/unload
 
 	XSLFI_RIFF_HEADER_60_BIT,                     ///< Size field in RIFF chunk header is 60 bit
 	XSLFI_HEIGHT_8_BIT,                           ///< Map tile height is 8 bit instead of 4 bit, but savegame version may be before this became true in trunk