After receiving funding from the Northern Ireland Assembly and issuing a tender, NI Railways awarded the contract to CAF, which already had a successful track record with the Class 3000 units.

The Class 4000 trains entered service in 2011, with significant internal differences from the C3K fleet, such as increased seating capacity and improved fuel economy.

Prior to 2000, the NIR rolling stock consisted of a mixture of diesel multiple unit types that had entered service between 15 and 25 years previously.

This was a like-for-like replacement meaning that older rolling stock had to be retained, and NIR could not introduce the service enhancements it desired.

[citation needed] The expanding economy led to increasing pressure to improve the rail network, with recommendations made in a debate in the Northern Ireland Assembly in May 2007.

The go-ahead was given on the publication of the draft budget of the Department for Regional Development, which allocated £137 million over three years to Translink, including for 20 trains.

The timetable for the introduction was released at the same time as the ITT:[12][13][10] Although the trains are externally similar to the C3K fleet, internally they have significant differences.

Of the seven extra sets, NIR has earmarked five for running in six-car formations, with the other two planned to improve service frequency on the Derry-Londonderry Line once renovation work has been completed and a new crossing loop laid.