A Deep Extractor for Visual Rail Surface Inspection

Rail surface inspection using a visual inspection system is an important task in the maintenance of railway networks. Recent inspection approaches have attempted to harness low-level features to identify visual defects. These methods' main issues are the limitations of low-level features and prior information about rail surfaces that show a wide variety of visual appearances on dynamic backgrounds. To overcome these problems, we propose a deep extractor (DE) that combines the strengths of fully convolutional networks and conditional random fields (CRFs), so that the deep network can learn the abstract features needed for the specified inspection task. Specifically, motivated by fully convolutional networks, a bilateral fully convolutional network containing two branches is proposed here: one encode-decode branch focuses semantic meaning; the other decode-encode branch encodes information on tiny defects. The two branches are aggregated to obtain a high-level feature map. Furthermore, the mean-field inference of a dense CRF with Gaussian pairwise potentials formulated as a recurrent neural network (CRF-RNN) is introduced to contribute to smoothing constraints and obtaining a fine-grained inspection result. By doing so, it can avoid offline post-processing and further lead the whole deep architecture to achieve end-to-end deep learning. Compared with classical approaches, our approach outperforms state-of-art approaches in analyzing two publicly available datasets.