conventional railroads; however a number of widely accepted variables have been acknowledged by the industry in recent years. Generally, high-speed rail is defined as having a top speed in regular use of over . Although almost every form of high-speed rail is electrically driven via overhead lines, this is not necessarily a defining aspect and other forms of propulsion, such as diesel locomotives, may be used. A definitive aspect is the use of continuous welded rail which reduces track vibrations and discrepancies between rail segments enough to allow trains to pass at speeds in excess of . Track radius will often be the ultimate limiting factor in a train's speed, with passenger discomfort often more imminent than the danger of derailment. Depending on design speed, banking, and the forces deemed acceptable to the passengers, curves often exceed a 5 kilometer radius. Tilting trains have been developed for achieving greater comfort for passengers, so higher speeds are possible on curvy tracks. Although a few exceptions exist, zero grade crossings is a policy adopted almost worldwide, with advanced switches utilizing very low entry and frog angles. Magnetic levitation trains fall under the category of high-speed rail due to their association with track oriented vehicles; however their inability to operate on conventional railroads often leads to their classification in a separate category. In the United States, the federal law has used the term "high-speed rail" as the rail services with "reasonably expected to reach sustained speeds of more than 125 miles per hour [200 km/h]" since 1998. In 2009, the United States Department of Transportation created a vision plan for national high-speed rail network with conflicting definitions by describing the lowest speed range of the high-speed rail systems as "Emerging HSR" with top speeds between and This created confusion in terminology and the media started to differentiate the "higher-speed rail" from the high-speed rail. Some state-level departments of transportation and council of governments now use different sets of definitions. For examples, North Central Texas Council of Governments uses the definition of the speeds over , and Texas Department of Transportation and Oklahoma Department of Transportation use the speeds of or more to define high-speed rail. These agencies have a separate category for higher-speed rail which can be a wide range of speeds between and . Very high-speed rail Generally, the speed range for very high-speed rail is between and . The term is