"Doctor Yellow" is the nickname given to the high-speed test trains that were used to monitor the conditions of track and overhead lines. The trains are based on their normal passenger carrying counterparts and look very similar to them, with line inspections usually being carried out at full speed.
The "Doctor" part of their name comes from their role of monitoring the track and overhead lines, while the "Yellow" comes from their bright yellow coloration. The original color scheme of yellow with blue stripes came from reversing the colors on cape gauge (3 ft 6 in, 1,067 mm) track recording cars, with their color scheme being blue with a yellow stripe.
Running schedules for Doctor Yellow trains are not made public.