 The source of the defect remains unknown, but may have been caused initially by a "cold shut", where the molten metal does not fuse together during metal casting.It often occurs when two waves of fast cooling cast iron meet, but why it should have occurred only about from one end remains unknown.It could have grown with repeated loading from passing traffic, and Tyler thought it lucky that it had failed when a goods train was passing, rather than a passenger train, when casualties would have been inevitable.