Question: which Amtrak's flagship service along the Northeast Corridor is  currently the only services in the country

Context paragraphs:
Northeast Corridor Line: The Northeast Corridor Line is a commuter rail operation run by New Jersey Transit along Amtrak's Northeast Corridor in the United States.  It is the successor to Pennsylvania Railroad trains between Trenton Transit Center and New York Penn Station.  After arrival at New York Penn Station, some trains load passengers and return to New Jersey, while others continue east to Sunnyside Yard for storage.  Most servicing is done at the Morrisville Yard, at the west end of the line.  The Northeast Corridor Line is colored red on New Jersey Transit system maps and its symbol is the State House.  The Princeton Branch is a shuttle service connecting to the line.

Mystic station (Connecticut): Mystic is an intercity train station on the Northeast Corridor, located off Roosevelt Avenue (US 1) east of downtown Mystic, Connecticut.  It is served by a limited number of trains on Amtrak's "Northeast Regional" service, with three to five daily trains in each direction.  Mystic is one of only three stations on the Northeast Corridor (along with adjacent stations Westerly and Kingston to the north) to be served exclusively by Amtrak, with no commuter rail service.

Amtrak's 25 Hz traction power system: Amtrak's 25 Hz traction power system is a traction power grid operated by Amtrak along the southern portion of its Northeast Corridor (NEC): the 225 route miles (362 km) between Washington, D.C. and New York City and the 104 route miles (167 km) between Philadelphia and Harrisburg, Pennsylvania.  The Pennsylvania Railroad constructed it between 1915 and 1938.  Amtrak inherited the system from Penn Central, the successor to Pennsylvania Railroad, in 1976 along with the Northeast Corridor.  This is the reason for using 25 Hz, as opposed to 60 Hz – which is the standard for power transmission in North America.  In addition to serving the NEC, the system provides power to New Jersey Transit Rail Operations (NJT), the Southeastern Pennsylvania Transportation Authority (SEPTA) and the Maryland Area Regional Commuter Train (MARC).  Only about half of the system's electrical capacity is used by Amtrak.  The remainder is sold to the commuter railroads who operate their trains along the corridor.

Amfleet: Amfleet is a fleet of single-level intercity railroad passenger cars built by the Budd Company for American company Amtrak in the late 1970s and early 1980s.  Budd based the Amfleet design on its earlier Metroliner electric multiple unit.  An initial order for 57 cars in 1973 to supplement the Metroliners on the Northeast Corridor grew to two orders totaling 642 cars, sufficient to reequip all the services on the Northeast Corridor and many other routes around the United States.  The first 492 cars, known as Amfleet I and completed between 1975–1977, were designed for short-distance service.  A second order of 150 cars, known as Amfleet II and completed between 1980–1983, were designed for long-distance service.  They were the last intercity passenger cars built by Budd.

Acela Express: The Acela Express ( ; colloquially abbreviated to Acela) is Amtrak's flagship service along the Northeast Corridor (NEC) in the Northeast United States between Washington, D.C. and Boston via 14 intermediate stops including Baltimore, Philadelphia, and New York City.  The route contains segments of high-speed rail, and Acela Express trains are the fastest trainsets in the Americas; they attain 150 mph on 28 mile of the route.

Night Owl (train): The Night Owl was a passenger train operated by Amtrak on the Northeast Corridor between Washington, D.C., and Boston, Massachusetts, via New York City.  It operated from 1972 to 1995 on an overnight schedule with sleeper service; it was the only such train on the Northeast Corridor.  In 1995 Amtrak dropped most individual train names from its Northeast Corridor services and the "Night Owl" became another "NortheastDirect" service, but still on an overnight schedule.  Amtrak replaced it with the "Twilight Shoreliner" in 1997.

High-speed rail in the United States: Plans for high-speed rail in the United States date back to the High Speed Ground Transportation Act of 1965.  Various state and federal proposals have followed.  Despite being one of the world's first countries to get high-speed trains (the Metroliner service in 1969), it failed to spread.  Definitions of what constitutes high-speed rail vary, including a range of speeds over 110 miles per hour and dedicated rail lines.  Inter-city rail in the United States with top speeds of 90 mph or more but below 125 mph is sometimes referred to as higher-speed rail.  The Acela Express (reaching 150 mph) and the Northeast Regional (reaching 125 mph) are currently the only services in the country (the Metroliner was discontinued in 2006).

Keystone Service: Amtrak's 195-mile (314 km) Keystone Service provides frequent higher speed passenger train service along the Amtrak Philadelphia to Harrisburg Main Line between the Harrisburg Transportation Center in Harrisburg, Pennsylvania and 30th Street Station in Philadelphia.  Most trains continue along the Northeast Corridor (NEC) to Pennsylvania Station in New York.

Northeast Corridor: The Northeast Corridor (NEC) is an electrified railway line in the Northeast megalopolis of the United States.  Owned primarily by Amtrak, it runs from Boston through Providence, New York City, Philadelphia, and Baltimore to Washington, D.C..  The NEC closely parallels Interstate 95 for most of its length, and is the busiest passenger rail line in the United States by ridership and service frequency as of 2013.  Branches to Harrisburg, Pennsylvania, and Springfield, Massachusetts, though not considered part of the Northeast Corridor, see frequent service from routes that run largely on the corridor.

Philadelphia to Harrisburg Main Line: The Philadelphia to Harrisburg Main Line is a rail line owned and operated by Amtrak in the U.S. state of Pennsylvania.  This is the only electrified Amtrak service outside of the main line of the Northeast Corridor.  The line runs from Philadelphia, where it meets the Northeast Corridor at Zoo interlocking (MP 1.9), west to Harrisburg (MP 104.6), where electrification ends.  It is part of the longer Keystone Corridor, which continues west to Pittsburgh along the Norfolk Southern Railway's Pittsburgh Line.  This section is sometimes referred to as "Keystone East" and is part of Amtrak's Keystone Service.

Supporting paragraphs: High-speed rail in the United States Acela Express
Answer: Acela Express