For the first time at the Riga Technical University a study was carried out of a highly defective frog core of grade 1/9. In the course of the research an analysis of crossing piece defects on the Latvian railway was carried out in eight railway sections during years of 2015, 2016, 2017. The defect of the frog core of grade 1/9 (the 60 E1 DO 07 12 frog type) was considered according to the basic classification of the defects, and the analysis and research of the cause of its fracture were conducted from the bolt hole to the web and the base of the frog core. The research process consisted of four stages: - Stage 1: determination of metal hardness according to the Brinell scale with a modern device “Tinius O Olsen” Firmware Version 1.07, FH - 31 Series. The obtained results should be compared with the data of the manufacturer's factory – Dowlais Steel. - Stage 2: determination of the chemical composition of the rail steel of the frog core 1/9 (in the rail top, rail web, and rail base) using the ARC-MET 8000 Mobile Lab Optical Emission Spectometer Analyser. The obtained data should be compared with the manufacturer's data. - Stage 3: determination of the rail steel structure. Drawing of main conclusions about the quality of the rail steel of the frog core of grade 1/9. - Stage 4: drawing of main conclusions and summary of the cause and development of the fracture from the bolt hole of the frog core of grade 1/9.