As CSX Transportation continues talks with Pan Am Railways to acquire its 1,700 miles of track in New England — including 121 miles in New Hampshire — Norfolk Southern Railway has told federal regulators it will oppose the transaction, which it claims will stifle competition, with adverse impacts on other railroads and commercial shippers in New England.

Pan Am and Norfolk Southern are partners in a $140 million joint venture known as Pan Am Southern, which provides Norfolk Southern access to New England by way of trackage rights on the 437 miles of the “Patriot Corridor” between Mechanicsville, N.Y., and Ayer, Mass.

Earlier this month, the counsel for Norfolk Southern, Michael Rosenthal of Covington & Burling, reminded the Surface Transportation Board — the federal agency responsible for regulating rail freight carriers — it approved the joint venture in 2009 as a means of enhancing competition among freight carriers in New England.

He told the board, approval of the acquisition would result in “competitive harm.”

In New Hampshire, Pan Am owns or operates five railway lines. The Pan Am Railways Main Line runs 35 miles between Plaistow and Rollinsford, carrying the Amtrak Downeaster between Boston and Portland, Maine, with stops in Exeter, Durham and Dover. The New Hampshire Main Line stretches more than 40 miles from Nashua to Concord and carries three-quarters of all freight (by weight) reaching the state by rail.

Perhaps most significantly, the main line runs within the Capitol Corridor, the preferred route for extending passenger rail service from greater Boston to Nashua, Manchester and perhaps Concord.

— MICHAEL KITCH