The 145km/h three-car trains were ordered in March 2009 and are a development of the class 3000, 23 of which were supplied by CAF to Translink between 2002 and 2004. The £114m class 4000 contract includes an option for 20 intermediate vehicles, which would lengthen the trains to four cars.

The class 4000 is five tonnes lighter than its predecessor and features a driver assist system for more efficient acceleration and braking, fuel-efficient ZF transmission, and MTU 6H1800R83 diesel engines with reduced exhaust gas emissions to comply with EU Stage IIIA standards.

Each train accommodates 212 passengers, 12 more than the class 3000, and there are new multi-purpose areas for pushchairs and bicycles.

A £27.8m depot for the new trains at Adelaide in south Belfast is under construction and will be completed later this year.

The final class 4000 will enter service in October, marking the completion of the programme to renew and expand Northern Ireland's dmu fleet. Passenger numbers on the Translink NI Railways network have risen 71% since 2002 and last year reached 10.7 million, the highest level since 1967.