Subways are an important part of the transport system in large cities. Damage to load-bearing structures of underground tunnels under fire for a long time disrupts the functioning of the transport pipeline. In this regard, to the structural elements lining tunnels there are imposed high requirements for fire resistance of 60-90 minutes (for escalators, subway tunnels and rail tunnels) and to 180 minutes for road tunnels. The purpose of research, the results of which are shown in the article, is the provision of fire resistance and lining of underground tunnels, made of iron tubing. The analysis of domestic and foreign experience in the field of fire resistance of cast iron structures for different purposes and different operating conditions was conducted. Initial and boundary conditions for the production and the estimated task of forming a mathematical model were formulated. Calculations of warm lining from cast iron tubing when exposed to the standard temperature of fire, depending on the thickness of construction, taking into account the heat transfer from the unheated side were conducted. Based on the results of the calculations it was determined the need for a fire-retardant action. A method of fire protection constructions, which allows you to keep the original tunnel section, was suggested. The essence of the method lies in the structural protection of structural iron tubing using fire-retardant mineral woolplates. The next stage of research was to conduct calculations of the regiment of heating the lining from cast iron tubing when exposed to the standard temperature of fire mode when using the proposed fire protection options. The calculation results have shown high efficiency of mineral wool plates use as fire protection of the iron tunnel lining. Calculations have shown that even under the worst conditions of heat irradiation into the ground it’s possible to provide the limit of fire resistance of cast iron tubing on the bearing capacity for at least 90 minutes. The results of the research show that the approach to defining the limit of fire resistance of structural metal tubing tunnels based on the achievement of critical temperature, requires experimental confirmation in tests of representative fragments of tunnels under the strain and conditions of heat irradiation into the ground close to the real. In the case of the transition to a flexible standardization in the transition to the real temperature of fire regimes on the object in question will also require further experimental work.