The Shinkansen network of high-speed bullet trains in Japan has a great safety record, thanks in large part to Doctor Yellow, the Shinkansen test train.
The Doctor Yellow Shinkansen is so named because it literally diagnoses any problems with the track and overhead wires as it speeds along, and is painted yellow instead of the regular white to make the trains more visible at night.

However, the color yellow also symbolizes happiness in Japan, and many Japanese locals believe that a rare sighting of this distinctive diagnostic train makes you happy and brings good fortune.
Doctor Yellow is also the fastest vehicle operating on the Shinkansen bullet train network, and often reaches its full speed of up to 443 km/h in order to properly test the tracks.  
Doctor Yellow functions
Doctor Yellow is regularly deployed on Japan?s rail lines in order to ensure that the standard gauge track used for Shinkansen services is sufficiently level and stable.
Shorter than most Shinkansen trains at only 7 cars long, Doctor Yellow contains sophisticated monitoring equipment that analyses track alignment and wear in overhead wiring.

It usually carries a 9-strong crew, consisting of 2 drivers, 3 track technicians, and 4 workers to analyse power-related issues, who monitor the data collected and relay any concerns to maintenance crews to repair the problem.
There are 2 types of inspection methods employed by Doctor yellow:

The “Nozomi inspection“, which runs on the high speed ...