1. American Public Transportation Associationhttp://www.apta.com/resources/statistics/Documents/FactBook/2014-APTA-Fact-Book.pdf
2. USA Federal Railroad Administrationhttp://safetydata.fra.dot.gov/OfficeofSafety/publicsite/Query/AccidentByRegionStateCounty.aspx
3. Mobile mapping;Kutterer,2010