B Line
B Line, B-Line or Line B may refer to the following:
Transportation
US
- B (New York City Subway service), a subway route in New York
 - B (SEPTA Metro), one of three subway lines in Philadelphia.
 - B-Line (Norfolk Southern), a freight-rail line in Virginia
 - Green Line B branch, a light-rail line in Boston, Massachusetts 
- B-Line Rivalry, a sports rivalry between Boston College and Boston University
 
 - B Line (Los Angeles Metro), a rapid transit line in Los Angeles County, California
 - B (Los Angeles Railway), former streetcar service in Los Angeles, US
 - RapidRide B Line, a bus route in King County, Washington
 - B Line (RTD), a light-rail system in Denver, Colorado
 - B Line (Valley Metro Rail), a light rail line in Phoenix, Arizona
 - Butte Regional Transit, a bus system in Butte County, California
 - Metro B Line (Minnesota), a planned rapid bus line in Minneapolis-Saint Paul 
- Riverview Corridor, previously known as the B Line
 
 - B (AC Transit), a bus route in the San Francisco Bay Area
 
Elsewhere
- Line B (Buenos Aires Underground), an underground line in Buenos Aires, Argentina
 - B-Line (Hamilton), a proposed light-rail line in Hamilton, Ontario
 - Line B (Prague Metro), a subway route in Prague, Czech Republic
 - B-Line (Sydney), a bus-rapid-transit system in Sydney, Australia
 - B-Line (Vancouver), a bus-rapid-transit system in Vancouver, British Columbia
 - Mexico City Metro Line B, a rapid-transit line in Mexico City
 - fr:Ligne B du métro de Rennes, a future subway route in Rennes, France
 
Other uses
- "B Line" (song), a track on Lamb's album Fear of Fours
 - The Eighties Matchbox B-Line Disaster, an English rock band
 - Index register, in computing, per outdated British use
 - A finding in lung ultrasound