Teng Cheong that day. The success of the 1977 Leyland Atlantean buses led to SBS purchasing another 500 Leyland Atlantean AN68 buses from 1978 up to 1984 while also trialing several demonstrators such as the Volvo B55, Dennis Dominator, Scania BR112DH and Volvo B10MD Citybus concurrently. The next batch of double-deck buses was introduced from 1984 to 1986 and consists of the Mercedes-Benz O305 and the Leyland Olympian, both of which were bodied by Walter Alexander Coachbuilders with an R-type body. In 1993, the first air-conditioned (as opposed to the average length then) double-decker bus, the Leyland Olympian 3-axle was launched as the "Superbus". The original 200 "Superbuses" were followed by an additional 471 Volvo Olympian 3-axle "Superbuses" and 100 non air-conditioned Volvo Olympian buses from 1994 to 2000. The Volvo B10TL, the first stepless, ultra-low-floor "Superbus", was launched in 1999 while 20 low-floor Dennis Trident 3 buses followed in 2001. The first wheelchair-accessible double-decker buses, the Volvo B9TL, were introduced in 2006; by 2014, SBS Transit had over 1631 of these buses in service. SBS Transit's monopoly on double-decker buses in Singapore ended in 2014 when rival SMRT Buses ordered 201 Alexander Dennis Enviro500 buses. It was followed up with 16 MAN ND323F A95 buses in 2015. Some of the existing double-deckers owned by SMRT Buses and SBS Transit were transferred to new operators Tower Transit and Go-Ahead under the Bus Contracting Model. Subsequent orders of B9TL and A95 buses are being made by the Land Transport Authority, are painted in a lush green livery (rather than the operator-specific liveries of SMRT and SBS Transit), and are used by all operators. Currently, there are more than 3,000 double-decker buses in Singapore, all of which are air-conditioned and wheelchair-accessible. Newer units introduced from 2018 onwards are also Euro VI-compliant and are equipped with USB charging ports and Visual Passenger Information Displays. In March 2017, the first three-door, two-staircase 12.8m long double-decker bus in Singapore was introduced by Tower Transit with the registration plate SG5999Z for a six-month trial period. After the trial, the bus was transferred to SBS Transit. The trial was successful and the Land Transport Authority purchased another 100 3-door double decker buses of the Alexander Dennis Enviro500 and MAN A95 builds in April 2019. In April 2018, SBS Transit introduced into service a single Volvo B8L registered as SG4003D. Fully electric Yutong E12DD double-decker buses will also