An idea to write this article emerged from the statistics of tracing defects of joints welded with a mobile rail welding machine type No 4 in the Latvian railway during years of 2014, 2015, 2016 and 2017. Said article will discuss a study of the welded joints of rail profile 60 E1, often used both in main line tracks and branch lines of the Latvian railway. The study includes an analysis of defects in the welded rail profile 60 E1 joints in the Latvian railway in eight sections of the line. A question emerges regarding the quality of joints fixed by flash-butt welding by means of a mobile rail welding machine. Once the metal hardness, chemical composition and structure of the rail steel is determined, the quality of flash-butt welding processed joints by means of the mobile rail welding machine type No 4 will be established. The study process comprises four phases: The first phase, determination of Brinell hardness by means of the modern device “Tinius O Olsen” Firmware Version 1.07, FH - 31 Series. The obtained results must be compared with data provided by the manufacturing plant Nizhniy Tagyl. The second phase, determination of the chemical structure of the welded joint of 60 E1 rail steel (in top, web, base) by means of the optical emission spectrometer ”PMI – Master PRO”. The obtained data must be compared with the manufacturer's data. The third phase, determination of rail steel structure. To draw main conclusions regarding the rail steel of welded joints by means of the mobile rail welding machine type No 4. The fourth phase, drawing of key conclusions and opinion regarding the quality of joints processed with flash-butt welding technique by means of the mobile rail welding machine type No 4.