Actually it is not needed. The airport reached its legal capacity of 18,000,000 in 2019 some two years before the Dart will be finished. The Dart will be needed if the Government approves the DCO application to be submitted next year. If it is refused the Council and LLAL will have blown £243m of Council Taxpayers money, as the Dart will not add a single passenger to the airport.
Work on the Dart should only be started on approval of the DCO and by the new concessionaire as part of their agreement and not by LLAL. Also remember that the existing terminal station will become an intermediate stop and that the running gear and electronic equipment will all be in the wrong place so the Dart will have to be shut down at some point to extend it.