The T3 (Chester Avenue line) is a trolley line operated by the Southeastern Pennsylvania Transportation Authority (SEPTA) that connects 13th Street Station in downtown Philadelphia, Pennsylvania, with Yeadon and Darby, Pennsylvania. It is one of five lines that are part of the Subway-Surface Trolley system.

Starting from its eastern end at the 13th Street, the T3 runs in a tunnel under Market Street. It makes stops at underground stations at 15th Street, 19th Street, 22nd Street, 30th Street, and 33rd Street. From 15th to 30th Streets, it runs on the outer tracks in the same tunnel as SEPTA's Market–Frankford Line.

Passengers may transfer free of charge to the Market–Frankford Line at 13th, 15th, and 30th Streets and to the Broad Street Line at 15th Street. Connections to the SEPTA Regional Rail are also available. Underground passageways connect the 13th and 15th Street Stations to Jefferson Station and Suburban Station.

Route T3 surfaces at the 40th Street Portal near 40th Street and Baltimore Avenue (US 13), and then runs southwest along Woodland Avenue shortly before moving to Chester Avenue. An alternate trolley line existing along 42nd Street itself joins the T3 line for several blocks. This line is used when tunnels are closed for maintenance or other unforeseen shutdowns. The line runs on a bridge over the Media/Wawa Line at 49th Street, however, it does not serve as an official stop for the trolley as the trolley stop is at the intersection of 49th Street and Chester Avenue just east of the station. A track along 49th Street allows for T3 service cutbacks, and is the pull-in/pull-out route for the T4 to access Baltimore Avenue a few blocks north.

Mt. Moriah Cemetery cuts off Chester Avenue at 61st Street, so the line makes a left turn at 60th Street then returns to the southwest at Kingsessing Avenue. At the southwest corner of the cemetery, the line stops at the Mt. Moriah Loop, and some use the loop for trolleys that do not go to either Yeadon or Darby. For those that do, the trolleys continue west on Kingsessing Avenue until it reaches 65th Street where it turns northwest. At Chester Avenue, 65th Street is staggered and the line navigates a left hand S-turn to rejoin 65th Street north of Chester Avenue. The line continues northwest along 65th Street and crosses over the Cobbs Creek Parkway, as well as Cobbs Creek which separates Philadelphia and Delaware Counties. It then immediately turns onto another section of Chester Avenue, on its way to the Yeadon Loop. Fernwood-Yeadon (SEPTA station) is further north of this terminus and has no connection. However the SEPTA Suburban Bus Routes 68 and 108, which do lead to Fernwood-Yeadon cross the line east of Yeadon Loop at the Church Lane intersection.

West of Yeadon Loop, some trolleys continue along Chester Avenue until it reaches its terminus at Cedar Avenue, where westbound and eastbound tracks split along 10th Street and 9th Street respectively. The tracks on 10th Street run straight, while those on 9th Street briefly run along Summit Street. At Main Street, where 10th Street ends, the westbound tracks turn left and proceed south to the next block at Darby Transit Center, which is primarily served by the T4 trolley, as well as numerous Suburban Division buses. Although service ends at Darby Transit Center, the tracks continue east along Main Street to access Elmwood District along the tracks of the T4.

Trolley service on the T3 originally operated from Yeadon to Front & Chestnut Streets via Chestnut and Walnut Streets. Service was rerouted into the Subway-Surface Tunnel on September 9, 1956.^{[citation needed]}

Service to was extended to Darby following the discontinuation of the Route 62 Darby–Yeadon shuttle trolley on January 24, 1971.^{[citation needed]}

In 2021, SEPTA proposed rebranding their rail transit service as "SEPTA Metro", in order to make the system easier to navigate. Under this proposal, the subway–surface lines were be rebranded as the "T" with a green color and numeric suffixes for each service. What was known as Route 13 was be renamed "T3".^[3][4] SEPTA described that "most comments were positive" in the public comment period for this rebranding project.^[5]

All are in the City of Philadelphia or the boroughs of Yeadon or Darby.

Neighborhood/ location	Station or stop	Connections	Notes
Market East	13th Street	SEPTA Metro: SEPTA City Bus: 27, 31, 32	Closed between 12:30–5:00am
Penn Center	15th Street/City Hall	SEPTA Regional Rail: all lines (at Suburban) SEPTA Metro: SEPTA City Bus: 4, 16, 17, 27, 31, 32, 33, 38, 44, 48 SEPTA Suburban Bus: 124, 125	Late night terminus
	19th Street	SEPTA Metro: SEPTA City Bus: 17, 31, 38, 44, 48, 62, 78 SEPTA Suburban Bus: 124
Center City West	22nd Street	SEPTA Metro: SEPTA City Bus: 7, 31, 44, 62 SEPTA Suburban Bus: 124, 125	Replaced 24th Street station
University City	Drexel Station at 30th Street	Amtrak (at 30th Street) NJ Transit: ACL Atlantic City Line (at 30th Street) SEPTA Regional Rail: all lines (at 30th Street) SEPTA Metro: SEPTA City Bus: 31, 49, LUCY	No direct passage to 30th Street Station
	33rd Street	SEPTA Metro: SEPTA City Bus: 30, 31, 49, LUCY	Serves Drexel University
	36th–Sansom	SEPTA Metro: SEPTA City Bus: 21	Serves University of Pennsylvania
	37th–Spruce	SEPTA Metro: SEPTA City Bus: 40, 42, LUCY	Serves University of Pennsylvania, Children's Hospital of Philadelphia
Spruce Hill	40th Street Portal	SEPTA Metro: SEPTA City Bus: 30, 40, 42, LUCY	End of T2 concurrency
	Chester & Woodland		Discontinued; end of T4 & T5 concurrency
	41st & Chester (EB)		Discontinued
	42nd & Chester	SEPTA City Bus: 30
	43rd & Chester
	45th & Chester
	46th & Chester
Squirrel Hill	47th & Chester
	48th & Chester
	49th & Chester	SEPTA Regional Rail: Media/Wawa Line (at 49th Street) SEPTA City Bus: 64
Kingsessing	51st & Chester
	52nd & Chester
	53rd & Chester
	54th & Chester	SEPTA City Bus: 52
	55th & Chester
	56th & Chester
	57th & Chester
	58th & Chester	SEPTA City Bus: G
Mount Moriah	59th & Chester
	Chester & 60th
	60th & Kingsessing
	61st & Kingsessing
	62nd & Kingsessing
	Mount Moriah
Elmwood	65th & Kingsessing
	Chester & 65th
	Belmar & 65th
	Windsor & 65th
	Cobbs Creek & 65th
Yeadon	65th & Chester
	Church & Chester	SEPTA City Bus: 68 SEPTA Suburban Bus: 108
	Yeadon & Chester
	Duncan & Chester
	Stetser & Chester
	Yeadon
	Alfred & Chester		Limited service
	Cedar & Chester		Limited service
Darby	Cedar & 9th (EB) Mulberry & 10th (WB)		Limited service
	Summit & 9th (EB) Wycombe & 10th (WB)		Limited service
	Summit & 10th (WB) Summit & 9th (EB)		Limited service
	Ridge & 10th (WB) Ridge & 9th (EB)		Limited service
	Main & 10th (WB)		Limited service
	Darby Transit Center	SEPTA Metro: SEPTA Suburban Bus: 113, 114, 115	Limited service

• Official SEPTA Route 13 schedule and map