The Rail and Maritime Transport Union, which had a fractious relationship with former chief executive Greg Miller, welcomed Reidy's appointment. Miller resigned suddenly in November last year.
Union general secretary Wayne Butson said Reidy would provide a steady hand on the tiller and stability at a challenging time for KiwiRail.
He said Reidy had a good understanding of rail.
"We have worked positively with Mr Reidy in the past and intend to do so in the future."
Butson said the "high performance high engagement" model had been introduced to KiwiRail during Reidy's previous time in the job, and had been part of improving relations between workers and management.
Reidy said much had changed since he was last at KiwiRail "especially with the level of Government investment which is enabling the purchase of two new state-of-the-art Interislander ferries, new locomotives for the South Island and infrastructure projects that are enhancing the network across the country and especially in Auckland".
Reidy's start date has yet to be advised. David Gordon would remain as acting group chief executive and Helen Rogers as acting deputy chief executive in the meantime.