The Department for Transport has provided £1.8m for the work, Network Rail said, with the extension currently underway.
It is hoped that passengers will be able to depart from all 12 cars next year once further work is carried out.
The work is part of a £22m, nine day rail closure, some of which is enabling work for the upgrade of Parsonage Road level crossing to a full barrier CCTV crossing to improve safety.
The work will also allow the recontrol of the Littlehaven signal box to the Three Bridges Rail Operating Centre (ROC) in June 2022.
Rail replacement buses are in place to help passengers continue to travel during the closure, which is in place until Monday.