Velocity is constant when the middle of the roller coaster is parallel with the ground (when the tangent of the curve the roller coaster is on has a slope of 0 at the center of mass of the roller coaster).
Twister - roller coaster - was created in 1999.
Sleeping on a Roller Coaster was created in 1992.
Tremors - roller coaster - was created in 1999.
Stealth - roller coaster - was created in 2006.
Rattler - roller coaster - was created in 1992.
Hades - roller coaster - was created in 2005.
Whirlwind - roller coaster - was created in 1993.
Thunderbird - roller coaster - was created in 2006.
Demon - roller coaster - was created in 1976.
Whizzer - roller coaster - was created in 1976.
Balder - roller coaster - was created in 2003.