Another warehouse fire struck the Lego Group in 1960, consuming most of the company's inventory of wooden toys. Godtfred decided that the plastic line was strong enough to abandon the production of wooden toys. As a result, Godtfred's brothers Gerhardt (then head of wooden toys) and Karl Georg left the Lego company and began a separate company called "Bilofix." By the end of the year, the Lego Group was employing more than 450 people. In 1961, Lego wished to expand sales to North America but did not have the logistical capabilities to do so. Lego made an arrangement allowing Samsonite to begin producing and selling Lego products in the United States and Canada. 1961 and 1962 saw the introduction of the first Lego wheels, an addition that expanded the potential for building cars, trucks, buses, and other vehicles from Lego bricks. Also, during this time, the Lego Group introduced toys targeted explicitly towards the pre-school market. In 1964, cellulose acetate, the material used to create Lego bricks, was replaced by the more stable acrylonitrile butadiene styrene (ABS plastic), which is still in use today. ABS is non-toxic, less prone to discoloration and warping, and is more resistant to heat, acids, salt, and other chemicals. Samsonite manufacturing in North America did not switch at the same time, and still used some degree of cellulose acetate in its Lego products. 1964 was the first year that saw the inclusion of instruction manuals in Lego sets. A notable Lego Group series, the Lego train system, was released in 1966. The original train sets included a 4.5-volt (which was changed to a 12-volt version two years later) motor, battery box, and rails. On 7 June 1968, the first Legoland Park was opened in Billund. This theme park featured elaborate models of miniature towns built entirely from Lego bricks. The three-acre (12,000 m2) park attracted 625,000 visitors in its first year alone. During the next 20 years, the theme park grew to more than eight times its original size and eventually averaged close to a million paying visitors per year. Sales of Lego sets reached more than eighteen million units in 1968. In 1969, the Duplo system went on sale. Duplo bricks are much larger than Lego bricks, making them safer for young children, but the two systems are compatible: Lego bricks can be fitted neatly onto Duplo bricks, allowing a seamless transition to the