The Guardian Angels began in February 1979 (the group was started by Curtis Sliwa in 1977 as the “Magnificent 13") and patrolled the #4 Lexington Avenue subway train. “The ‘Magnificent 13,’ a Bronx-based group that patrols a subway train they call the ‘muggers’ express’” was cited in an Associated Press story on February 18, 1979. The nickname “muggers’ express” was probably coined by Sliwa to describe what was then New York’s most dangerous subway line.
The subways became safer and the nickname “muggers’ express” (or “mugger’s express") is mostly of historical use.
Wikipedia: 4 (New York City Subway service)
The 4 Lexington Avenue Express is a rapid transit service in the A Division of the New York City Subway. It is colored apple green on station signs, route signs, and the official subway map, since it uses the IRT Lexington Avenue Line in Manhattan.
The 4 operates at all times. The daytime service pattern is between Woodlawn in the Bronx and Utica Avenue in Crown Heights, Brooklyn via Jerome Avenue Local in the Bronx, Lexington Avenue Express in Manhattan, and Eastern Parkway Express in Brooklyn.
Google News Archive
18 February 1979, Tuscaloosa (AL) News, “Anti-crime crusaders picket gang movie” (AP), pg. 12A, col. 5:
Saturday’s demonstration at the Times Square theater showing the film was carried out by members of the “Magnificent 13,” a Bronx-based group that patrols a subway train they call the “muggers’ express.”
Volume 113, Part 2
Wearing red berets as a badge of office, the original 13 went into action last Feb. 13, with three teams patrolling what is known as the Muggers’ Express, the No. 4 IRT train from Woodlawn in The Bronx to Atlantic Avenue in Brooklyn.
Google News Archive
11 April 1979, The Gazette (Montreal, Quebec), pg. 92, col. 1:
THEY’RE MAKING THE ‘MUGGERS’ EXPRESS’ SAFE
NEW YORK—(SPECIAL)—Inside a car of the subway “Muggers’ Express,” swaying as it moved along under Harlem, a drunk slipped off his seat and landed on the floor.
The Guardian Angels
By James Haskins
Hillside, NJ: Enslow Publishers
Patrols from eight p.m. to four a.m. began on the so-called “Muggers’ Express,” the No. 4 IRT (Interboro Rapid Transit) line from Woodlawn Avenue in the Bronx down the length of Manhattan Island to Atlantic Avenue in Brooklyn.
Crime, Fear, and the New York City Subways:
The Role of Citizen Action
By Dennis Jay Kenney
New York, NY: Praeger
For passengers on the Number 4 and Number 2 lines, there is undoubtedly an unsettling feeling as they wait for trains Curtis Sliwa has publicly named the “Mugger’s Express” and “The Beast.”
Maynard and Jennica
By Rudolph Delson
Boston, MA: Houghton Mifflin Company
They used to call the 4 train “the Muggers’ Express.”
24 Things You Didn’t Know About New York City Subways
PUBLISHED ON FEBRUARY 26, 2014 BY BIG APPLED EDITORIAL TEAM
10. The 4 train was once considered the worst train to be on in the 70s and 80s, especially during the night. People referred to it as “Mugger’s Express.” Crime became so bad that people took matters into their own hands to form the Guardian Angels.