The article focuses on the original author's development – the flexible liner for a rail tank car that allows reducing the risk of leakage of hazardous liquid cargos (in particular crude oil or petroleum products) during their transportation by rail. Application of the liner allows ensuring hermetical containment of a tank car even if a barrel has any minor defects, excludes a contact of cargo with tank walls, which leads to increase in the security of hazardous liquid cargo transportation. This original development enables ensuring the safety and security of hazardous cargo transportation and minimizing its impact on the environment. The liner can also be used for increasing the service life of a rail tank car and reducing the costs of its maintenance.