Question: Into what company did the railroad which had operated the Twin Cities Zephyr merge in 1970?

full_context: Document 1: Ozark State Zephyr: The Ozark State Zephyr was a streamlined passenger train operated by the Chicago, Burlington and Quincy Railroad (CB&Q) and the Alton Railroad (the "Alton") between St. Louis and Kansas City in Missouri, home of the Ozarks. It operated from 1936 to 1939. The "Zephyr" was one of several short-haul Midwestern routes operated by the CB&Q's then-revolutionary articulated streamlined trainsets, including one of the original "Twin Cities Zephyrs".
Document 2: Northern Transcon: The Northern Transcon, a route operated by the BNSF Railway, traverses the most northerly route of any railroad in the western United States. This route was originally part of the Chicago, Burlington and Quincy Railroad, Northern Pacific Railway, Great Northern Railway and Spokane, Portland and Seattle Railway systems, merged into the Burlington Northern Railroad system in 1970.
Document 3: Nebraska Zephyr: The Nebraska Zephyr was a streamlined passenger train operated by the Chicago, Burlington and Quincy Railroad (CB&Q, commonly known by the shorter name of "Burlington") between Chicago, Illinois, Omaha, Nebraska and Lincoln, Nebraska. It operated from 1947 to 1971. Until 1968 the two original "Twin Cities Zephyr" articulated trainsets, the "Train of the Gods" and "Train of the Goddesses", operated the service and became synonymous with it. The "Nebraska Zephyr" was one of many trains discontinued when Amtrak began operations in 1971. The "Train of the Goddess" set is preserved at the Illinois Railway Museum in Union, Illinois.
Document 4: Ak-Sar-Ben Zephyr: The Ak-Sar-Ben Zephyr was a streamlined passenger train operated by the Chicago, Burlington and Quincy Railroad (CB&Q) between Lincoln, Nebraska and Chicago, Illinois, United States. The first version operated from 1940 to 1947; a revived service operated from 1953 to 1970. The "Ak-Sar-Ben" portion of name was created by spelling Nebraska (Neb-ras-ka) backwards and taken from a fraternal organization of the same name. "Zephyr" was a name applied by the CB&Q to many of its trains, beginning with the "Pioneer Zephyr" in 1934. The name derived from "Zephyrus", the Greek god of the west wind.
Document 5: Chicago Subdivision: The Chicago Subdivision or Chicago Sub is a railroad line in Illinois that runs about 38 mi from Chicago to Aurora and hosts Metra's BNSF Railway Line commuter service. It is operated by BNSF Railway as the easternmost part of the railroad's Northern Transcon to Seattle, Washington. This line is known as the Racetrack because it is mostly triple-tracked and supports fairly fast trains. It had been operated by a BNSF ancestor, the Chicago, Burlington and Quincy Railroad, which introduced high-speed "Zephyr" passenger trains in 1934 and ran many of them along this subdivision from Chicago to points west.
Document 6: Wayzata Subdivision: The Wayzata Subdivision or Wayzata Sub is a railway line that runs about 93 mi from Willmar to Minneapolis, Minnesota. Currently operated by BNSF Railway, this was part of the Great Northern Railway's transcontinental line from Minneapolis to Seattle, Washington. Today, BNSF's Northern Transcon travels up the Staples Subdivision instead, which is a more direct route to Fargo, North Dakota. As of May, 2011, the route is of moderate capacity and typically sees around 15 trains per day.
Document 7: St. Paul Subdivision: The St. Paul Subdivision or St. Paul Sub is an 30.9 mi railway line running from Minneapolis through neighboring Saint Paul, Minnesota and on into Saint Paul's southeastern suburbs along the Mississippi River. It is a segment of BNSF Railway's Northern Transcon which runs from Chicago, Illinois to Seattle, Washington. Between Minneapolis and Saint Paul themselves, this route runs on former Northern Pacific Railway trackage, and now forms the northern set of BNSF tracks running between the two cities. The companion route running slightly to the south is the Midway Subdivision, though the St. Paul Subdivision is the busier set of tracks, hosting about 59 trains per day as of April 2009. It is the second-busiest rail line in the state, after the segment of the Staples Subdivision between Fridley and Coon Rapids.
Document 8: Chicago, Burlington and Quincy Railroad: The Chicago, Burlington and Quincy Railroad (reporting mark CBQ) was a railroad that operated in the Midwestern United States. Commonly referred to as the Burlington or as the Q, the Burlington Route served a large area, including extensive trackage in the states of Colorado, Illinois, Iowa, Kansas, Kentucky, Missouri, Montana, Nebraska, Wisconsin, Wyoming, and also in New Mexico and Texas through subsidiaries Colorado and Southern Railway, Fort Worth and Denver Railway, and Burlington-Rock Island Railroad. Its primary connections included Chicago, Minneapolis-St. Paul, St. Louis, Kansas City and Denver. Because of this extensive trackage in the midwest and mountain states, the railroad used the advertising slogans "Everywhere West", "Way of the "Zephyrs"", and "The Way West". It merged into Burlington Northern in 1970.
Document 9: Olympian Hiawatha: The Olympian and its successor the Olympian Hiawatha were passenger trains operated by the Chicago, Milwaukee, St. Paul and Pacific Railroad (the "Milwaukee Road") between Chicago and the Pacific Northwest. The "Olympian" operated from 1911 to 1947 and was, along with its running mate the "Columbian", the first all-steel train to operate in the Pacific Northwest. The streamlined "Olympian Hiawatha" operated from 1947 to 1961 and was one of several Milwaukee Road trains to carry the name "Hiawatha." The "Olympian Hiawatha" was designed by industrial designer Brooks Stevens and included the distinctive glassed-in "Skytop" observation-sleeping cars. It later featured full-length "Super Dome" cars.
Document 10: Twin Cities Zephyr: The Twin Cities Zephyr was a streamlined passenger train on the Chicago, Burlington and Quincy Railroad (CB&Q), running between Chicago and the Twin Cities of Minneapolis and Saint Paul in Minnesota. It was the second "Zephyr" service introduced by CB&Q after the record-setting Denver–Chicago "dawn to dusk dash" of the "Pioneer Zephyr" trainset.
Document 11: Denver Zephyr: The Denver Zephyr was a streamlined passenger train operated by the Chicago, Burlington and Quincy Railroad between Chicago, Illinois, and Denver, Colorado. It operated from 1936 to 1973. The "Denver Zephyr" continued operating after the Burlington Northern Railroad merger in 1970. BN conveyed the train to Amtrak in 1971; Amtrak would merge it with the Chicago–Oakland "San Francisco Zephyr" and drop the name in 1973.
Document 12: Zephyr Rocket: The Zephyr Rocket was an overnight passenger train operated jointly by the Chicago, Burlington and Quincy Railroad ("Burlington Route") and the Chicago, Rock Island and Pacific Railroad ("Rock Island Lines") between Saint Louis, Missouri and the Twin Cities of Saint Paul and Minneapolis, Minnesota, with major intermediate stops in Burlington, Cedar Rapids, and Waterloo, Iowa. The Chicago, Burlington & Quincy carried the train between St. Louis and Burlington, while the Chicago, Rock Island and Pacific carried it between Burlington and Minneapolis/St. Paul. Motive power and equipment were pooled and traveled the entire distance without change.
Document 13: Zephyrette: A Zephyrette was a hostess on the "California Zephyr" between 1949 and 1970, while the train was jointly operated by the Chicago, Burlington and Quincy Railroad, the Denver and Rio Grande Western Railroad, and the Western Pacific Railroad. The position was the brainchild of Velma McPeek, the Burlington's Supervisor of Passenger Train Services, and was part of an effort to attract families to rail travel and accommodate their needs. Zephyrettes first appeared on the "Denver Zephyr" in 1936, and shortly thereafter on the "Twin Cities Zephyr", but the position was discontinued during World War II. Zephyrettes resumed service after the war in 1949, but only on the "California Zephyr".
Document 14: Twin Cities Hiawatha: The Twin Cities Hiawatha, often just Hiawatha, was a named passenger train operated by the Chicago, Milwaukee, St. Paul and Pacific Railroad (also known as the Milwaukee Road), and traveled from Chicago to the Twin Cities. The original train takes its name from the epic poem "The Song of Hiawatha" by Henry Wadsworth Longfellow. There are a number of Hiawatha-themed names within the city of Minneapolis, the terminus of the original train. The first "Hiawatha" ran in 1935; in 1939 the Milwaukee Road introduced a second daily trip between Chicago and Minneapolis. The two trains were known as the Morning Hiawatha and Afternoon Hiawatha, or sometimes the "AM Twin Cities Hiawatha" and "PM Twin Cities Hiawatha". The Milwaukee Road discontinued the "Afternoon Hiawatha" in 1970 while the "Morning Hiawatha" continued running until the formation of Amtrak in 1971.
Document 15: Midway Subdivision: The Midway Subdivision or Midway Sub is a 12.4 mi railway line in Minneapolis and Saint Paul, Minnesota on the BNSF Railway's Northern Transcon which runs from Chicago, Illinois to Seattle, Washington and Portland, Oregon. This is former Great Northern Railway trackage, and now forms the southern set of BNSF tracks running between the two cities. The companion route running slightly to the north is the St. Paul Subdivision, former Northern Pacific Railway tracks. The Midway Subdivision hosted about 24 trains per day as of September 2015.
ground_truth: Burlington Northern