English: 2012 Alexander Dennis Enviro 400 (CF) body with a Scania N230UD chassis and a Scania DC9 (N94UD) 6.7 litre inline 4 turbo diesel engine and battery power, Reg.SN12AVX of Sullivan Buses (E48) (ex-Tower Transit) at Arnos Grove Underground Station, 6 April 2023.

Walter Alexander designed the ALX400 bus body for use with the Dennis Trident 2, Volvo B7TL (designed by Leyland and initially built in the UK) and DAF DB250 chassis, production commencing in 1998. In 2000, Alexander merged with Dennis and Plaxton (which had bought Northern Counties and parts of Duple) to form Transbus and the Alexander ALX400 was renamed the Transbus ALX400.

As Alexander and Dennis were now part of the same company, the ALX400 when using the Dennis Trident chassis was effectively a semi-integral bus, although the Volvo and DAF chassis remained options. In 2004 Transbus was put into administration and was subject to a management buyout, the company becoming Alexander Dennis Ltd (or ADL) and the ALX400 bus was renamed accordingly.

Production ceased in 2006. However, the replacement was the ADL Enviro 400 which was an update of the ALX400 and available as an integral bus (although the chassis was still branded as a Trident 2 for a few years!) or with Volvo, Scania or MAN chassis.