I'll try and list the major ones and what happened to them.
Over a long time, the Union Pacific absorbed the Southern Pacific (which had itself bought the Denver & Rio Grande Western) along with its subsidiary St. Louis Southwestern ("Cotton Belt"), the Missouri Pacific (which had itself bought the Texas & Pacific), the Western Pacific, the Chicago & North Western (which had itself bought the Chicago Great Western) and the Missouri-Kansas-Texas.
The Burlington Northern was formed by a merger of the Chicago, Burlington & Quincy (which owned the Colorado & Southern), the Northern Pacific, the Great Northern and the Spokane, Portland & Seattle. It later absorbed the St. Louis-San Francisco ("Frisco"). Eventually, it merged with the Atchison, Topeka & Santa Fe to form Burlington Northern Santa Fe.
The Norfolk & Western absorbed the New York, Chicago & St. Louis ("Nickel Plate Road"), Wabash, Virginian and the Akron, Canton & Youngstown. Then it merged with Southern Railway (which owned the Central Railroad of Georgia and had already bought the original Norfolk Southern) to form a new Norfolk Southern.
The Chessie System was a merger of the Baltimore & Ohio, Chesapeake & Ohio (which had absorbed the Pere Marquette) and Western Maryland. Meanwhile, the Atlantic Coast Line and Seaboard Air Line merged into the Seaboard Coast Line. The SCL merged with the Louisville & Nashville (which had absorbed the Monon, formerly known as Chicago, Indianapolis & Louisville) to form the Family Lines. Later, the Family Lines absorbed the Clinchfield Railroad and became the Seaboard System. Seaboard System changed its name to CSX Transportation shortly before absorbing the Chessie System. CSX later absorbed the Richmond, Fredericksburg & Potomac.
The Erie Railroad and the Delaware, Lackawanna & Western merged into the Erie-Lackawanna. The Pennsylvania Railroad and the New York Central merged into Penn Central. Very shortly after, the PC absorbed the New York, New Haven & Hartford. PC and EL, along with the Reading Railroad, the Central Railroad of New Jersey, the Lehigh Valley, and the Lehigh & Hudson River, merged into Conrail. Eventually, Conrail broke up and was absorbed by CSX and NS.
Kansas City Southern now owns Grupo Transportacion Ferroviaria Mexicana (TFM) and the Texas-Mexican Railway.
Canadian Pacific absorbed the Toronto, Hamilton & Buffalo, the Minneapolis, St. Paul & Sault Ste. Marie ("Soo Line", which had already bought the original Wisconsin Central and the Milwaukee Road) and the Delaware & Hudson (after Gulford cast it off; which see.)
At various times, Canadian National absorbed the Illinois Central (which had merged with the Gulf, Mobile & Ohio and been known as Illinois Central Gulf for a while), Grand Trunk Western (which had absorbed the Detroit, Toledo & Ironton), the Northern Alberta Railway, the British Columbia Railway (formerly known as Pacific Great Eastern), the new Wisconsin Central, the Bessemer & Lake Erie, the Duluth, Missabe & Iron Range, the Duluth, Winnipeg & Pacific and the Central Vermont.
The Maine Central, Boston & Maine and Delaware & Hudson became the Guilford System, but it cast off the D&H very soon. The D&H, as noted above, is now owned by CP.
The Chicago, Rock Island & Pacific went bankrupt, as did the Lehigh & New England. The Rutland broke up into shortlines.
Some regionals from the steam era, such as the Florida East Coast, the New York, Susquehanna & Western, the Lake Superior & Ishpeming, the Chicago, South Shore & South Bend, and the Elgin, Joliet & Eastern, have survived.
One of the most important modern regionals is Montana Rail Link.
Most passenger service is no longer handled by the freight railroads. The main long-distance passenger companies are Amtrak in the US and VIA Rail in Canada.
The last narrow-gauge common carrier in the US, the White Pass & Yukon, is now a tourist line. The last such railroad in North America, CN's subsidiary Newfoundland Railway (officially known as TerraTransport in its last years) was abandoned.
The "7 main lines" today are CN and CP in Canada, BNSF and UP in the western US, CSX and NS in the eastern US, and KCS in the central US.
! Thanks again, I really appreciate this info. Rob