D Line
- D (Los Angeles Railway)
- D (New York City Subway service)
- D (SEPTA Metro)
- D Line (Los Angeles Metro)
- D Line (RTD)
- D Train (disambiguation)
- D-line (IRC)
- D-line, Port Harcourt
- Fraunhofer lines
- Green Line D branch
- Line D (Buenos Aires Underground)
- Line D (Prague Metro)
- Line D (Rome Metro)
- Metro D Line (Minnesota)
- RapidRide D Line