Funding for this research was provided by:
National Natural Science Foundation of China (71171164)
Scientific research Foundation of Shaanxi Railway Institute (KY2016-04)
Text and Data Mining valid from 2019-08-01
Received: 29 January 2019
Accepted: 17 July 2019
First Online: 1 August 2019
: The author declares that he has no competing interests.