From New York City to Washington DC
US Bus Station.com, Your Local Bus and Coach Route Database

Departs are usually from NYC:
 125 Park Ave. btw. 41st & 42nd Streets
 155 Empire Blvd. cor. Bedford Ave
 122 Allen St. cor Delancy Street
 303 West 34th St. cor. 8th Ave
 Midtown - Penn Station/28 Allen St (corner of Canal St. and Allent St.)
To Union Station, 15th St NW & Dupont Circle.

Most of buses are express and have wi-fi on board!
Departs are every hour from 1.30am to 9.30pm, depends of the peak hours.
It is advisable to arrive at least 15 minutes before departing.
Average Cost: From 17$ to 22$ one way
Average Duration: Between 4hours and 5hours