{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,5]],"date-time":"2026-10-05T19:24:57Z","timestamp":1791228297909,"version":"4.1.0"},"reference-count":140,"publisher":"MDPI AG","issue":"14","license":[{"start":{"date-parts":[[2026,7,21]],"date-time":"2026-07-21T00:00:00Z","timestamp":1784592000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100012226","name":"Fundamental Research Funds for the Central Universities","doi-asserted-by":"publisher","award":["2026JBMC055"],"award-info":[{"award-number":["2026JBMC055"]}],"id":[{"id":"10.13039\/501100012226","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Shanghai Key Laboratory of Rail Infrastructure Durability and System Safety","award":["202502"],"award-info":[{"award-number":["202502"]}]},{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["52307001"],"award-info":[{"award-number":["52307001"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]},{"award":["52307001"],"award-info":[{"award-number":["52307001"]}],"id":[{"id":"https:\/\/ror.org\/01h0zpd94","id-type":"ROR","asserted-by":"publisher"}]},{"name":"State Grid Electric Power Engineering Research Institute Co., Ltd.","award":["52500126000L"],"award-info":[{"award-number":["52500126000L"]}]},{"DOI":"10.13039\/501100004826","name":"Beijing Natural Science Foundation","doi-asserted-by":"publisher","award":["2605024"],"award-info":[{"award-number":["2605024"]}],"id":[{"id":"10.13039\/501100004826","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Young Elite Scientists Sponsor-ship Program of the Beijing High Innovation Plan","award":["0250748"],"award-info":[{"award-number":["0250748"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>With increasing rail traffic intensity, reliable detection of shallow subsurface rail damage is essential for operational safety. This critical narrative review evaluates non-destructive testing technologies relevant to defects whose active crack front or principal scattering zone lies within the upper approximately 0.5\u201310 mm of the rail, while treating the 10\u201315 mm range as a transition to deeper-defect verification. Magnetic flux leakage, magnetic particle inspection, visual inspection, eddy current testing, and conventional ultrasonic testing are first examined as screening or confirmatory comparators. The review then focuses on four ultrasonic surface-wave excitation routes\u2014contact piezoelectric, active air-coupled, electromagnetic acoustic, and laser ultrasonic\u2014and distinguishes source-specific laboratory capability from demonstrated field evidence. Because the cited studies use different defect geometries, rail conditions, sensor configurations, speeds, and decision criteria, their numerical values are reported as source-conditioned evidence rather than as a normalized ranking. An engineering decision matrix links defect depth and size, inspection speed, surface condition, and noise environment to a recommended screening\u2013confirmation workflow. The synthesis identifies contact piezoelectric UT\/PAUT as the most mature quantitative confirmation route, while EMAT, air-coupled UT, and laser UT retain method-specific advantages but require stronger natural-defect and in-service validation.<\/jats:p>","DOI":"10.3390\/s26144614","type":"journal-article","created":{"date-parts":[[2026,7,21]],"date-time":"2026-07-21T15:17:52Z","timestamp":1784647072000},"page":"4614","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Non-Destructive Testing Technology for Shallow Subsurface Defects in Rails: A Review with Focus on Ultrasonic Surface Wave Methods"],"prefix":"10.3390","volume":"26","author":[{"given":"Tianyu","family":"Song","sequence":"first","affiliation":[{"name":"School of Physical Science and Engineering, Beijing Jiaotong University, Beijing 100044, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7315-2692","authenticated-orcid":false,"given":"Lisha","family":"Peng","sequence":"additional","affiliation":[{"name":"State Key Laboratory of Power System Operation and Control, Tsinghua University, Beijing 100084, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Songling","family":"Huang","sequence":"additional","affiliation":[{"name":"State Key Laboratory of Power System Operation and Control, Tsinghua University, Beijing 100084, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Zijing","family":"Huang","sequence":"additional","affiliation":[{"name":"State Key Laboratory of Power System Operation and Control, Tsinghua University, Beijing 100084, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2854-044X","authenticated-orcid":false,"given":"Qibo","family":"Feng","sequence":"additional","affiliation":[{"name":"School of Physical Science and Engineering, Beijing Jiaotong University, Beijing 100044, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7090-6719","authenticated-orcid":false,"given":"Hongyu","family":"Sun","sequence":"additional","affiliation":[{"name":"School of Physical Science and Engineering, Beijing Jiaotong University, Beijing 100044, China"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,7,21]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Ji, K., Wang, P., and Jia, Y. 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