Question: What authority manages the regional passenger train service that runs through the same junction as West Amesbury Branch Railroad?

Context paragraphs:
Rapido (train): Rapido was the brand name for the Canadian National Railway's (CN) express passenger train service in the Quebec City–Windsor Corridor.  "Rapido" service was introduced on October 31, 1965 and lasted until the end of CN passenger train service with its transfer to VIA on October 29, 1978.

Z-Train: The Z-Train was the original name of a proposed passenger train service that would operate primarily on Union Pacific Railroad lines between Los Angeles Union Station, Ontario, California, and a new station to be built adjacent to the Las Vegas Strip, with a travel time of five to five-and-a-half hours.  Amtrak last operated passenger train service to Las Vegas in 1997 on the "Desert Wind".  The train would offer an alternative to traveling on congested Interstate 15.

Cariboo Prospector: The Cariboo Prospector or "Cariboo Dayliner" or The BC Rail Budd cars was a passenger train service in British Columbia, Canada, which used Budd Rail Diesel Car trains.  It was operated by the Pacific Great Eastern, later known as the British Columbia Railway Company and then BC Rail.  The train ran from BC Rail's North Vancouver railway station, the one located a few blocks from the current North Vancouver railway station used by the "Rocky Mountaineer", and ran to Lillooet railway station.  From there a section was split from the train that would continue down to Prince George BC Rail station located in BC Rail's Prince George yards.  This train service ended along with the other BC Rail passenger services in 2002.  A section serving the line between Lillooet, Seton Portage, and D'Arcy was replaced by the Koaham Shuttle.

Enola Branch: The Enola Branch is a railroad segment of the Port Road Branch and it was a rail line; the Enola Branch railroad segment and the rest of the Port Road Branch is owned and operated by the Norfolk Southern Railway in the U.S. state of Pennsylvania.  The railroad segment runs from Washington Boro northwest to Marysville and it is a former Pennsylvania Railroad rail line.  Its south end is at a former junction with the Atglen and Susquehanna Branch, where the main segment of the Port Road Branch continues southeast.  Its north end is at the Pittsburgh Line.  Along the way, it meets the York Secondary at Wago Junction (near York Haven) and goes under the Lurgan Branch at Lemoyne.  Norfolk Southern labels the Enola Branch as part of the Port Road Branch, officially ending the Enola Branch's existence as a rail line, the main segment of the Port Road Branch runs from Marysville, Pennsylvania south to Perryville, Maryland.  The line goes through the Enola Yard.

Sunshine Special: The Sunshine Special was inaugurated by the St. Louis, Iron Mountain and Southern Railway, (later the Missouri Pacific Railroad), on December 5, 1915, to provide a premium level of passenger train service between St. Louis, Little Rock, and destinations in Texas.  Until being shifted into a secondary role by the "Texas Eagle" diesel powered streamliner in August 1948, the "Sunshine Special" served as the flagship of the Missouri Pacific Railroad's passenger train service.  The last vestige of the "Sunshine Special" name did not disappear from Missouri Pacific timetables until 1962.

Downeaster (train): The Downeaster is a 145 mi regional passenger train service, managed by the Northern New England Passenger Rail Authority (NNEPRA, created by the State of Maine), and operated by Amtrak.  Named for the Down East region of Maine, the train runs from North Station in Boston, Massachusetts, to Brunswick, Maine, with 10 intermediate stops.  The train also operates five daily round trips between Portland and Boston, three of which continue to Brunswick.

West Amesbury Branch Railroad: The West Amesbury Branch Railroad was a railroad that once led form Newton Junction, New Hampshire to Merrimac, Massachusetts.  Although the railroad does not exist, the Amtrak "Downeaster" line now passes through the Newton Junction station, which is now a pizza restaurant, and most of the railroad, is now a gravel walking trail.

Greenbrier Presidential Express: The Greenbrier Presidential Express was a luxury passenger train that was planned to operate between Union Station in Washington, D.C., United States, and the train station at the Greenbrier resort in White Sulphur Springs, West Virginia.  The project was initiated in June 2011 but canceled in May 2012 due to numerous issues, among which were capacity constraints on the Buckingham Branch Railroad and Federal approval of the train's engineering.

Basmane-Nazilli Regional: The Basmane-Nazilli Regional, numbered as B310, (Turkish: "Basmane-Nazilli Bölgeseli" ) is a 175 km passenger train operated by the Turkish State Railways, that runs between Basmane Terminal in Izmir to Nazilli.  The train operates along the same route as the Basmane-Denizli Regional, serving a shorter portion of the line.  Before regional service was extended to Denizli after track upgrades, three daily trains in each direction would operate as the Basmane-Nazilli Regional.  Train service started in 2009 after the completion of the Şirinyer Tunnel.

Rail replacement bus service: A rail replacement bus service uses buses to replace a passenger train service either on a temporary or permanent basis.  The train service that is replaced may be of any type such as light rail, tram, streetcar, commuter rail, regional rail or heavy rail, intercity passenger service.  The rail service may be replaced if the line is closed due to rail maintenance, a breakdown of a train, a rail accident, strike action, or if the rail service is not economically viable.

Supporting paragraphs: West Amesbury Branch Railroad Downeaster (train)
Answer: Northern New England Passenger Rail Authority