5:30
`` We must find him , '' she said . `` Something has happened to him . Did you think to ask Apgar if he remembered seeing Rex on his train Wednesday night ? '' Apgar was the conductor on the 5:30 express . `` No , I 'll go down to the station and ask him this afternoon before he goes out . '' Roy returned with the announcement that Apgar was sure Rex had not been on his train . `` Then there is only one other theory . ''