Continuous monitoring of transportation infrastructures has significantly contributed to the management of new systems, by taking advantage of new tools in the area of Operational Modal Analysis (OMA). It also plays a crucial role in ensuring the safety and structural integrity of old railway bridges. Traditional monitoring systems, while reliable, are often expensive, complex, and require significant maintenance, limiting their widespread application. In this context, MEMS (Micro Electromechanical Systems) accelerometers have emerged as a promising alternative, offering a cost-effective, robust, and autonomous solution for Structural Health Monitoring. Due to the growing importance of continuous monitoring of structures in Civil Engineering, this article aims to explore new measurement systems and sensor solutions for Structural Health Monitoring of bridges, using as a case study one of the steel bridges in the Beira Alta international railway line in Portugal. The main objective of this study is to evaluate the dynamic behaviour of the Trezói Bridge through modal operational analysis and assess the viability and suitability of MEMS accelerometers in a continuous monitoring system installed in a structure of this type. By considering the results of ambient vibration tests carried out with MEMS accelerometers, as well as the measurement of the bridge's dynamic behavior over an year of operation, this type of accelerometers demonstrated high reliability, autonomy, and effectiveness, overcoming key challenges associated with traditional monitoring solutions, such as cost and maintenance. This paper details this case study, exploring accessible, scalable, and sustainable solutions that contribute to infrastructure management with credible performance.