Wikipedia: Doctor Yellow is the nickname for the high-speed test trains that are used on the Japanese Shinkansen ("Bullet Train") dedicated express passenger train routes. The trains have special equipment on board to monitor the condition of the track and overhead wire, including special instrumented bogies and observation blisters.
The 'Doctor' part of the name is obvious from their test and diagnostic function, and the 'Yellow' part comes from the bright yellow color they are all painted. Some have a blue waistline stripe, some a green one. The original color scheme of yellow with a blue stripe (applied to the Class 921 track-recording cars) was created by reversing the colors (blue with yellow stripe) used on 1,067 mm (3 ft 6 in) narrow-gauge track-recording cars.
In build and appearance, they are very similar to production, passenger carrying Shinkansen trains, and line inspection is carried out at full line speed (i.e. up to 270 km/h/168 mph on the Tōkaidō Shinkansen).