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

full_context: Document 1: Penn Line: The Penn Line is a MARC commuter rail line running from Union Station, Washington D.C. to Perryville, Maryland via Penn Station, Baltimore, Maryland on Amtrak's Northeast Corridor. It is MARC's busiest and only electric line. Currently the line is the fastest commuter rail line in the country, with trains running at speeds of up to 125 mph . The service is operated under contract by Amtrak which supplies employees to operate trains, and maintains the right-of-way and MARC's electric locomotives and passenger cars. The line is administered by MARC, a service of the Maryland Transit Administration (MTA).
Document 2: 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.
Document 3: Railroad classes: In the United States, railroads are designated as Class I, II, or III, according to size criteria first established by the Interstate Commerce Commission (ICC) in 1911, and now governed by the Surface Transportation Board.
Document 4: Federal-Aid Highway Act of 1973: The Federal-Aid Highway Act of 1973 (Public Law 93–87; 87 Stat. 250) is legislation enacted by the United States Congress and signed into law on August 13, 1973, which provided funding for existing interstate and new urban and rural primary and secondary roads in the United States. It also funded a highway safety improvement program, and permitted states for the first time in U.S. history to use Highway Trust Fund money for mass transit. The law also established the first national speed limit (of 55 mph ).
Document 5: Atlantic City Express (Amtrak train): The Atlantic City Express was an Amtrak train that ran from both New York City and Washington, D.C. to Atlantic City, New Jersey, in the Northeastern United States. The train operated on the Northeast Corridor from New York City and Washington, D.C. to 30th Street Station in Philadelphia, Pennsylvania, where it went east to Atlantic City.
Document 6: 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).
Document 7: 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.
Document 8: Northeast Regional: The Northeast Regional is a higher-speed rail service operated by Amtrak in the Northeastern and Mid-Atlantic United States. In the past it has been known as the "NortheastDirect", "Acela Regional", or "Regional". It is the busiest Amtrak route, carrying 8.04 million passengers in fiscal year (FY) 2013, a 0.4% increase over the 8.01 million passengers in FY2012. The "Northeast Regional" service earned over $568.7 million in gross ticket revenue in FY2013, a 6.2% increase over the $536 million earned during FY2012.
Document 9: Jungbu Naeryuk Expressway: A speed zone exists from Exit 1 to Exit 13 (Masan-Gimcheon). The maximum speed is 100 km/h, and the minimum speed is 50 km/h. Another speed zone exists from north of exit 13 to exit 28 (Gimcheon-N. Yeoju). The maximum speed limit is 110 km/h, and the minimum speed limit is 50 km/h.
Document 10: Route of California High-Speed Rail: The California High-Speed Rail system will be built in two phases. Phase 1 will be about 520 miles long, and is planned to be completed in 2029, connecting the downtowns of San Francisco, Los Angeles, and Anaheim using high-speed rail through the Central Valley. In Phase 2, the route will be extended in the Central Valley north to Sacramento, and from Los Angeles east through the Inland Empire and then south to San Diego. The total system length will be about 800 miles long. Phase 2 has no dates as yet.
Document 11: 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.
Document 12: Standard gauge: The standard gauge (also Stephenson gauge named after George Stephenson, International gauge, or normal gauge) is a widely used railway track gauge. Approximately 55% of the lines in the world use this gauge. All high-speed rail lines, except those in Russia, Uzbekistan, Finland and Australia, utilize standard gauge. The distance between the inside edges of the rails is defined to be 1435 mm except in the United States, where it is still defined in Imperial and US customary units as approximately 4 ft 8½ in. It is also called the UIC gauge or UIC track gauge, the European gauge in the EU and Russia, or uniform gauge in Queensland, Australia.u
Document 13: Capital Express (Turkey): The Capital Express (Turkish: Başkent Ekspresi ) was one of the six daily intercity trains operating between Istanbul and Ankara on the Istanbul-Ankara railway before the Yüksek Hızlı Tren high-speed train service replaced all intercity trains on the line. The Capital Express was the fastest of the six trains, making limited stops only in large cities. The train would complete it's journey in just over four hours and in the Eskişehir Province, trains would reach conventional speeds of 150 km/h , which still hold the record for fastest conventional train service in Turkey. When the high-speed rail service was opened between Ankara and Eskişehir on 13 March 2009, the Capital Express, along with the other five intercity trains, were cut back between Istanbul and Eskişehir. During this period two more daily intercity trains were added, the Sakarya Express and Eskişehir Express, increasing daily intercity service to eight trains.
Document 14: 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.
Document 15: Amtrak: The National Railroad Passenger Corporation, doing business as Amtrak , is a passenger railroad service that provides medium- and long-distance intercity service in the contiguous United States. Founded in 1971 to take over most of the remaining U.S. passenger rail services, it is partially government funded yet operated and managed as a for-profit corporation.
ground_truth: Acela Express