1. 1. C. Campos-castellanos, Y. Gharaibeh, P. Mudge, V. Kappatos “The Application of long range ularasonic testing (LRUT) for examination of hard to access areas on railway tracks.” IEEE paper present in 5th IET conference on Railway condition monitoring and NDT.
   2. Shankar Kumar J, Rakshith Kaushik T R, Priyanka B, Sandhya Rani N, Shruthi P. “Automated railway track tracking system using ultrasonics” Department of Electronics & Communication in C.I.T, Tumkur, India and Department of Electronics & Communication in G.S.S.I.T, Bangalore, India. National Insruments tutorial on “Fundamentals of Ultrasonic Imaging and Flaw Detection” published on Feb. 11, 2010.
   3. Abdiweli Abdillahi Soufi, Abdirasoul Jabar Alzubaidi “Remote control system through mobile and DTMF” Information Technology College, Open University of Sudan Electronics Engineering School, Sudan University of Science and Technology. Elec Freaks “Ultrasonic Ranging Module HC - SR04”. “Integrated DTMF receiver MT8870D/MT8870D-1” by MITEL.
   4. Indian Railways Facts sheet on Indian railways – Safety Performance 2013 present in lok Shabha by Ministry of Railways, Government of India.
   5. M. Pradeep, K. Naganarasaiah Goud “Damage detection of railway track by sensor using advanced RISC machine” Department of E.C.E,Annamacharya Institute of Technology & Sciences, Rajampet, A.P, INDIA.