In the process of heat exchange with the external environment, the internal temperature of ballastless track structure presents a nonlinear distribution. The vertical temperature gradient will cause repeated warping and deformation of track slab, resulting in mortar layer separation, which will affect driving comfort and track durability. The traditional temperature field analysis method of concrete structure based on thermodynamics has the disadvantages of too many assumptions, difficult parameter selection and too much calculation of energy consumption. In this paper, based on the finite element software ANSYS, the heat exchange was transformed into the boundary condition of heat flux, which was applied to the thermodynamic analysis model to study the nonlinear temperature distribution law of ballastless track. The accuracy of the analysis method was verified by the measured data. On this basis, the regional distribution law of temperature gradient of ballastless track under different geographical coordinates and climatic conditions was studied. By adding a regional adjustment coefficient, the vertical temperature load model of ballastless track suitable for typical areas in China was proposed. The proposed temperature load model makes up for the lack of refinement of climate division and temperature load model in relevant specifications, and has strong engineering application and popularization value.

The track–bridge interaction is a fundamental concern in the field of railway engineering, which plays an important role in the optimization design of railway bridges, especially for heavy-haul railway and high-speed railway bridges. This paper systematically introduces the research status of the CWR track–bridge interaction for large-span bridges of high-speed railway in China. The evolution process of the track–bridge interaction model from the simplest elastic bar and linear longitudinal resistance model to the complex beam–rail interaction model considering the loading history is described. In this paper, the modeling methods of the track–bridge interaction model for five types of long-span railway bridges, namely simply supported beam bridge, continuous beam bridge, cable-stayed bridge, arch bridge, and suspension bridge, are systematically introduced, and the characteristics of longitudinal force distribution under the track–bridge interaction are analyzed. This paper discusses the practical application of the theory of the track–bridge interaction on extra-large-span bridges from the aspects of system dynamic performance evaluation and system safety evaluation. The practical application of track–bridge interaction theory under special conditions such as earthquake load, complex temperature load, shrinkage and creep load, and superposition of multiple loads is emphasized. It provides guidance for the further improvement of the track–bridge interaction model and the design of large-span high-speed railway bridges in the future.

In order to investigate the stress characteristics of ballastless track under high latitude, and multi-source and multi-field extreme temperature conditions. Based on the finite element theory and the elastic foundation beam–plate principle, a finite element model of the ballastless track considering the limit convex abutment, gel resin, and interlayer bonding is established. The mechanical characteristics of the ballastless track under the slab–CAM layer bonding state, mortar separation, freeze–thaw degradation and forced deformation of the foundation are studied. Considering the deterioration of materials, the bending moment and reinforcement of track structures in cold regions are checked and calculated. The studies show that under the action of negative temperature gradient load, the edge of the track slab is subjected to tension, and structural separation occurs at the edge of the slab. When the interface between the track slab–CAM layer is poorly bonded, the bearing capacity can be improved, and the amount of separation can be reduced by increasing the structural stiffness of the CAM layer. Under the action of freeze–thaw cycles, the material performance deteriorates seriously, the separation between the track structures intensifies, the baseplate is seriously powdered and cracked, and the maximum tensile stress exceeds 6 MPa. The CAM layer and the baseplate are weak structures, and the foundation frost heave occurs at the expansion joint of the baseplate, which is the frost heave condition. Under freeze–thaw deterioration, the original reinforcement design of the substructure structure does not meet the requirements of structural cracks and reinforcement yield stress. In severely cold areas, the structural reinforcement scheme should be reasonably determined.

The Qinghai–Tibet railway is the plateau permafrost railway with the highest altitude and the longest line in the world. The natural conditions along the line are harsh, with many unfavorable factors such as low temperature, strong ultraviolet radiation, and large changes in daily temperature, resulting in frequent bridge damage. In order to study the bridge temperature field and its effect in the permafrost region of the Qinghai–Tibet railway, a long-term field test was carried out, and a calculation model of sunshine temperature field of concrete two-piece T-beam was established based on the principle of meteorology and heat transfer. On this basis, the beam temperature difference load mode, beam section temperature distribution law, and temperature effect were obtained. As revealed, the daily temperature difference of the Qinghai–Tibet railway bridge is large in winter, which is related to the ground’s effective radiation and surface reflectivity, and the maximum value has exceeded the current codes. The beam section temperature field shows “internal heat and external cold” in the morning and “internal cold and external heat” from noon to evening. Under the action of strong radiation and large temperature difference, bridge displacement occurs frequently, by which it is easy to cause damage to rail fasteners and bridge bearings. Based on the field test and finite element analysis, the bridge temperature difference load mode was proposed, which makes up for the deficiency that the relevant codes do not consider the plateau’s special climatic conditions and can provide a reference for the construction of plateau railways such as the Sichuan–Tibet railway.

We demonstrated a stabilized Brillouin random fiber laser based on external injection of the 2^nd order Stokes for the first time, benefiting a dynamic gain-loss balance for relative intensity noise suppression under optimized injection power.