Police say the suspect exposed himself to a woman at the 61st Street-Woodside Station on Jan. 3.
Bilal Johnson has been putting up stickers in the subway to protest police brutality, he said.
The sign is part of the agency's campaign aimed at reminding riders of subway etiquette.
The incident occured just before 7 p.m. Friday evening, FDNY confirmed.
B and D trains were rerouted because of a broken rail at Grand Street, accoridng to the MTA.
The 4, 5, 6, 7, A, C, E, F, N, Q and R lines are slated for service changes.
Service on the 3, 4, 5, 6, A, D, E, F, G, N and R trains will be affected this weekend, the MTA said.
The 2, 6, A, D, G, N, Q and R lines are slated for service disruptions.
Parts of the broken rail causing the derailment should have been repaired a year before the accident.
More than 2,000 additional people could ride the L train per hour with the improvements, the MTA said.
The assault happened near the Prospect Park stop on the B/Q line, police said.
The 29-year-old woman was thrown against a wall by a thief who was after her bag, police said.
A train malfunction at Grand Central Terminal disrupted 7 trains for Wednesday morning's commute.
Makeup artist Amanda Thesen shows how to get past the bad lighting and bumpy ride.
A hero barista, a late-night sauna party, oyster happy hours and more made news this week.
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