It is a diesel-hauled carriage service, introduced by the New Zealand Railways Department in 1964 after passenger demand between Masterton to Wellington exceeded the capacity of the diesel railcars then used.

They were fitted with fluorescent strip lighting similar to Northerner and Endeavour cars, and painted in a new, brighter shade of red, with white roofs as opposed to the standard silver oxide.

With the streamlining of Greymouth expresses into one out-and-back operation, one Picton car became surplus to requirements and joined the Masterton fleet.

In 2006, Hillside Engineering won the contract from the Greater Wellington Regional Council to rebuild 18 British Rail Mark 2 carriages to replace the fleet.

They are classified SW for cars with passenger saloon only, SWS with servery and SWG with luggage compartment and generator for power supply.

The S stands for "Scenic Series" and "W" for "Wairarapa", to distinguish them from their Capital Connection counterparts, classified S. On 11 May 2007, the first four cars entered service, with three more introduced on 18 June 2007 making up a 7-car consist.

The inaugural run of the first four cars was on 14 May 2007, being met by the Minister of Transport, Annette King, on arrival on Platform 9 at Wellington at 9.20 am.

The Wairarapa Connection is the only diesel-hauled service operated by Metlink, hauled by diesel-electric locomotives from KiwiRail's fleet.

Because of the short platforms at Renall Street, Solway, Matarawa and Maymorn stations, selective door operation is employed and passengers are asked to alight from the three northernmost (Masterton-end) carriages.

They made up a fourth set of carriages, increasing capacity and allowing more flexibility, but were criticised by for their smaller seat pitch, poor lighting, and lack of tray tables and power outlets.

[7] The pitch issues were rectified over the 2013 Christmas period by removing a row of seats and adjusting the location of the remainder, and additional tables fitted.

Those using ten-trip tickets and monthly passes must pay the cash fare difference as a surcharge, i.e. $5.00 for Petone and Waterloo, $1.00 for Upper Hutt.

This was temporarily removed in April 2011 to help alleviate a rolling stock shortage brought about by the delayed introduction of the Matangi electric multiple units,[9] and was later made permanent in October 2011.

Most have occurred at level crossings, where road vehicles have passed warning signs or signals and have been hit by the train.

On 17 October 1997, the mid-morning service to Wellington broke down 2.5 km into the Rimutaka Tunnel from the Featherston portal after an electrical fault in locomotive DC 4951's control gear.

[10] On 23 July 2009, the 17:33 service to Masterton hit a mudslide blocking the line just north of Maoribank tunnel near Maymorn.

Only minor injuries were reported, but the slip and the derailed rolling stock blocked the line, preventing services operating until Sunday evening (26 July).

[11][12] The accident highlighted a flaw in the design of the pneumatically-operated interior doors in the SW carriages, which stuck in position when the compressed air supply was lost as a result of the locomotive being shut down.

[14] [15] [16] In July, Labour leader Andrew Little promised expenditure on the line from the proposed regional development fund.

[18] In 2019, the GWRC proposed replacing in 2025 the Capital Connection and Wairarapa Connection trains with 15 four-car dual-mode multiple units, to operate from overhead power from Wellington to Upper Hutt or an on-board power source north of Upper Hutt; to cost $415 million.

[19][20] In February 2020, the GWRC announced that the Wairarapa Connection carriages were to be refurbished to extend their life until new rolling stock is introduced.