How find internals crack in railway track in running real time in working project?

Sir 

I am working on a project for "automatically railway track inspection device".This is a vehicle which run on railway track & continue inspection the track for any defect. In case of finding any defect a message is send to control room. So I choose some defect which can be find by my model.1. External cracks2. Internal cracks3. Track Joint condition
Now,Your help need to me for solve in "internal crack find in running time" which method is used.
Please, give your suggestion to me for improvement in my project & also for effective working.

I did a search on Google and found multiple ways of doing it; and the ultrasonic sensor looks like the most accurate one that detects cracks. You should refer to a couple of projects already published -

https://www.researchgate.net/publication/311862190_Automotive_Crack_Detection_for_Railway_Track_Using_Ultrasonic_Sensorz

https://ieeexplore.ieee.org/document/7847816/authors#authors

This is a working project:

https://www.youtube.com/watch?v=26saVwpDSHk

https://www.youtube.com/watch?v=26saVwpDSHk

I hope this helps. Feel free to ask more questions.