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. Five different Doctor Yellow trains have existed, although two are now out of service.
The trains have special equipment on board to test the condition of the track and overhead wire, including special instrumented trucks 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.
In build and appearance, they are very similar to production, passenger carrying Shinkansen trains, and they are capable of the same tremendous speeds; testing is often carried out at very high speed.
Test runs are normally carried out at night, when service trains do not run.
With thanks to byun byun Shinkansen at http://www.h2.dion.ne.jp/~dajf/byunbyun/