What type of train operates as a non-revenue service train?

A non-revenue service train is typically one that operates without generating fare income. These trains may be utilized for various purposes, such as maintenance, testing, or moving equipment.

The D Train, specifically, is a type of rapid transit train that may sometimes operate in this capacity for essential functions outside of standard passenger service. It is not always associated with carrying paying passengers; rather, it can serve administrative, logistical, or operational roles which do not contribute to fare revenue. This unique function aligns with the definition of a non-revenue service train, distinguishing it from express or passenger trains, which focus on transporting fare-paying customers, and freight trains, which are mainly classed in the context of transporting goods rather than transporting people, directly generating revenue as well.

In summary, the D Train can serve specific operational needs that do not involve carrying paying customers, thereby fitting the criteria for non-revenue service.