The DRA on the Armstrong Powerhouse Class 321.
The Driver's Reminder Appliance (DRA) is a manual switch in the driving cab of a passenger train. When operated it glows bright red and prevents the driver from being able to take power. It was introduced in the design and operation of United Kingdom passenger trains in the 1990s in response to a series of railway accidents where train drivers had passed a signal at danger when starting away from a station.
The DRA functions only in the driving cab from which the train is being driven, regardless of its state within other driving cab(s), for example, the Armstrong Powerhouse Class 321 and Class 90 packs.