Question: Arrow Dynamics started designing roller coasters with which pair of intertwined steel coasters at Disneyland?

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Matterhorn Bobsleds: The Matterhorn Bobsleds are a pair of intertwined steel roller coasters at Disneyland in Anaheim, California.  It is modelled after the Matterhorn, a mountain in the Alps on the border with Switzerland and Italy.  It is the first known tubular steel continuous track roller coaster.  Located on the border between Tomorrowland and Fantasyland, it employs forced perspective to seem more impressively large.

Ninja (Six Flags St. Louis): Ninja is an Arrow Dynamics/Vekoma steel roller coaster located in the Studio Backlot section of Six Flags St. Louis in Eureka, Missouri.  The ride features a loop, a sidewinder, and a double corkscrew.  It was originally built for and located at Expo '86 in Vancouver, B.C., operating as Scream Machine from May to October 1986.  It began operating as Ninja at Six Flags St. Louis in 1989.  The coaster was started by Arrow Dynamics, but when Arrow fell into bankruptcy, it was sold to Vekoma, who finished it.  The ride is formerly shared a name with Blue Hawk at Six Flags Over Georgia until that coaster's retheming in 2016.  Another roller coaster with the same name exists at Six Flags Magic Mountain, but that ride is an Arrow Dynamics suspended swinging coaster.  The Ninja, like most modern roller coasters, requires a minimum of two employees to dispatch the train.  One operates the main panel, which controls the restraints, gates, and has a section for the mechanics.  The other operator stands at the remote enable.  Both operators have to do an all clear sweep and thumbs up, then press their buttons at the same time.

Gemini (roller coaster): Gemini is a racing roller coaster with a wooden structure and steel track, located at Cedar Point in Sandusky, Ohio.  Built in 1978 by Arrow Dynamics and designed by Ron Toomer, it is one of the oldest roller coasters in the park.  When the ride opened, it was marketed as the tallest, fastest and steepest roller coaster in the world.  All three record-breaking claims were falsely made as other coasters around the world already beat Gemini.  The all-steel Loch Ness Monster at Busch Gardens in Williamsburg, Virginia, opened earlier that year before Gemini and was taller but had a shorter drop.  Screamin' Eagle at Six Flags St. Louis opened two years prior to Gemini and was faster at 62 mph.

Arrow Dynamics: Arrow Dynamics was a roller coaster design and manufacturing company based in Clearfield, Utah, United States.  Successor to Arrow Development (1946–1981) and Arrow Huss (1981–1986), which were responsible for some of the most influential advancements in the amusement and theme park industries.  Among the most significant was tubular steel track, which provided a smoother ride than the railroad style rails commonly used prior to the 1960s on wooden roller coasters.  The Matterhorn Bobsleds at Disneyland was Arrow's first foray into roller coasters, in 1959.  Arrow and their successors would continue to build trend-setting ride systems for the next 45 years.

Steel roller coaster: A steel roller coaster is a roller coaster that is defined by having a track made of steel.  Steel coasters have earned immense popularity in the past 50 years throughout the world.  Incorporating tubular steel track and polyurethane-coated wheels, the steel roller coasters can provide a taller, smoother, and faster ride with more inversions than a traditional wooden roller coaster.

Viper (Six Flags Magic Mountain): Viper is a steel roller coaster made by Arrow Dynamics of the United States.  The roller coaster is located in the Baja Ridge area of Six Flags Magic Mountain in Valencia, California.  Viper is the last of the three 7-looper roller coasters built by Arrow Dynamics to remain operating.  The other two, Shockwave at Six Flags Great America and the Great American Scream Machine at Six Flags Great Adventure, were demolished in 2002 and 2010, respectively.  The roller coaster replaced a HUSS ride type named Condor.

Roller coaster phobia: Roller coaster phobia is a colloquial and slang term describing an individual's fear of roller coasters.  While roller coasters are the most popular theme park attraction, certain people feel nauseated, afraid, dizzy, or unsafe when riding roller coasters.  In the majority of cases, this fear is related to acrophobia, claustrophobia, or a traumatic event.  Roller coaster phobia can be overcome relatively easily, the best cure being going on the tallest/fastest roller coaster before riding roller coasters of a lesser magnitude.  Roller coaster phobia has been linked to previously experiencing an accident, vertigo, and fear of heights.  In rare cases, riders may be afraid of roller coasters due to a painful ride, tight restraints, being thrown around in the restraints, and excessive g-forces on the body of the rider.

Phantom's Revenge: Phantom's Revenge (formerly known as Steel Phantom) is a steel roller coaster at Kennywood.  When it opened in 1991, it was the fastest roller coaster and had the longest drop of any roller coaster in the world.  The ride was originally manufactured by Arrow Dynamics.  After the 2000 season, D.H. Morgan Manufacturing was brought in to renovate the ride, most notably to remove the four inversions.  Unlike most roller coasters, the ride's second drop through Thunderbolt's structure is longer than its first drop.

Excalibur (Valleyfair): Excalibur is a steel roller coaster with a wooden structure located at Valleyfair in Shakopee, Minnesota.  It was built in 1989 by Arrow Dynamics, for the cost of $3,000,000.  The ride is 105 feet (32 m) tall with a top speed of 54.5 mph (87.7 km/h), and has a minimum height requirement of 48 inches.  Excalibur is only one of a few coasters in the world that has a steel track with a wood frame.  Two others, Gemini and Cedar Creek Mine Ride, are located at sister park Cedar Point in Sandusky, Ohio.  It follows a customized figure-eight track layout and is rather short for a major roller coaster, with a total ride duration of two minutes and thirteen seconds.  Most likely due to its location at the very western edge of the park and the more recently built roller coasters such as Wild Thing, Steel Venom and Renegade.

Hypercoaster: A hypercoaster is any roller coaster with a height or drop measuring greater than 200 ft .  The term was first coined by Arrow Dynamics and Cedar Point in 1989 with the release of the first full-circuit hypercoaster in the world, the Magnum XL-200.  Other roller coaster manufacturers developed their own models with custom names, including "Mega Coasters" from Intamin, "Hyper Coasters" from Bolliger & Mabillard, and "Hyper-Hybrid Coasters" from Rocky Mountain Construction.  The competition between amusement parks to build increasingly taller roller coasters eventually led to "giga coasters" which exceed 300 ft and "strata coasters" which exceed 400 ft .

Supporting paragraphs: Arrow Dynamics Matterhorn Bobsleds
Answer: Matterhorn Bobsleds