## 1 Introduction  
Railway bogies sit beneath railway cars and support the train's load while enabling acceleration and deceleration. The bogie frame is connected to electrical and mechanical equipment, including the transmission system, braking equipment, suspension equipment, and wheel set. The bogie controls the train speed and the passage rate at a curve, and influences the running stability. Bogie frames are generally comprised of metallic materials, including SM490A steel or high strength steel [1]. However, recently, fiber-reinforced composites have been used to reduce the structural weight of the bogie [2; 3; 4; 5; 6]. This decreased weight reduces the amount of energy that is required to run the vehicle. In addition, the maintenance costs of the rail tracks are reduced, owing to the reduced axle load. Fiber-reinforced composites have a higher specific strength and specific stiffness than metallic materials. Their durability and resistance to corrosion are also excellent. However, their design and production requires advanced technology, owing to the non-uniformity of the material, and the breakage mode complexity. Composite bogie frames are designed to have a complex internal structure, and all of the fabrication processes are performed manually and in sequence by workers. Hence, even if they are fabricated via qualified standard fabrication processes, there is a high probability that they will not have a uniform structural performance. Thus, there is a need for technology that can effectively monitor whether or not the composite structure meets the required performance design. Currently, based on finite element (FE) analysis results, only specific positions where deformation is expected are monitored, using a strain gauge during the load test. An optical fiber sensor has several merits, such as a small diameter (250 \(\upmu\)m), the ability to represent multi-point measurements as one line, and immunity from electromagnetic interference because there is no flow of free electrons. Recently, a fiber Bragg grating (FBG) sensor, a type of optical fiber sensor, has been studied broadly as a replacement for the strain gauge. Similar to the strain gauge, it can only be used in predetermined positions [7; 8; 9], but it also has a limited capacity to monitor the distributed strain of a structure.  
A method for monitoring structural deformation in composite bogie frames was developed for the first time using an optical fiber sensor system with a high spatial resolution based on Brillouin optical correlation domain analysis (BOCDA). In the vertical train load test, the distributed strain of the lower part of the composite side beam was measured using an attached 3 m-long optical fiber with a 3 cm spatial resolution. The result was compared with the FE analysis results to verify the design. In addition, an optical fiber was embedded inside the frame of the bogie during the fabrication process in order to monitor the frame's strain distribution during vertical train load tests. The strain distribution monitoring results can be used to verify the balance of the bogie, and to detect unpredictable concentrated loads due to internal defects produced during the production process.