maintaining the fleet of parking lot trams used for shuttling guests between the various theme park parking lots and their respective main entrances (except at the Magic Kingdom, where the trams drop guests off at the Transportation and Ticket Center). Because the trams require guests to transfer from one's wheelchair and to fold all strollers and wheelchairs before boarding, they are not ADA-accessible. Both the Magic Kingdom and Epcot parking lots have two tram lanes, with the Magic Kingdom trams serving the "Heroes" and "Villains" sides of the lot, while the Disney's Hollywood Studios and Disney's Animal Kingdom parking lots have only one tram lane. The original tram tractors, which ran on compressed natural gas (CNG), were built by United Tractor of Chesterton, Indiana in 1969, while the tram cars were built by Arrow Development during the same period. However, these tractors experienced many problems including overheating, transmission issues, as well as electrical and air brake troubles and were prone to frequent breakdowns. A new fleet of tram tractors, which were custom designed and built in-house by Disney, were put into service in 1972. These tram tractors also originally ran on CNG, but were converted to run on diesel fuel only a few years after entering service due to numerous problems encountered with using CNG. Over the years, these problems were slowly resolved and the tractors were eventually converted back to running on CNG starting in the late 1990s and into the early 2000s. Beginning in late 2010 and throughout 2011, safety doors were added to all of the tram cars along with outward facing speakers so that guests waiting to board the trams could hear the safety announcements more clearly. The 1972 tractors remained in service for over forty five years until late 2016, when Disney began testing a new tram tractor prototype at Disney's Animal Kingdom. The new tractor ran on propane, which made it quieter and more fuel efficient. Following successful testing, Disney upgraded the entire tram tractor fleet to the new propane-fueled tractors throughout 2017. Minnie Van The vehicle for hire service named after Minnie Mouse began testing in July 2017, with the first Minnie Van service being offered to guests staying at Disney's BoardWalk Resort, Disney's Yacht Club Resort, and Disney's Beach Club Resort at the end of that month. The service is now available to all visitors on Walt Disney World property, whether overnight