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For many historic engineering structures, including early 20th-century truss bridges, no comprehensive technical documentation has survived, making them highly vulnerable to irreversible loss. This study addresses this challenge by developing and testing a non-invasive, UAV-based photogrammetric methodology for the comprehensive documentation of the Niestępowo railway viaduct in northern Poland. A dense geodetic control network was established using GNSS and total station measurements, providing a metrically verified reference framework for 3D reconstruction. Two photogrammetric software platforms—Bentley ContextCapture and Agisoft Metashape—were employed and comparatively evaluated in terms of processing workflow, accuracy, and model fidelity. To ensure methodological robustness, both tools were used for cross-validation of the generated 3D models and for the comparative assessment of their dimensional consistency against archival documentation. The results confirm that both platforms can produce highly accurate, photorealistic 3D models suitable for engineering inventory and heritage preservation, with Agisoft Metashape yielding slightly higher geometric precision, while Bentley ContextCapture ensured superior automation for large datasets. The generated 3D models reproduced details such as rivets, cracks, and corrosion marks with millimeter-level accuracy. The presented workflow demonstrates the potential of UAV photogrammetry as a reliable and scalable method for safeguarding cultural and technical heritage. By enabling the creation of metrically precise digital archives of historic bridges, the methodology supports future conservation, monitoring, and restoration efforts—preserving not only physical form but also the historical and engineering legacy of these structures.