The decision is despite rumours the Government is looking to renationise rail by buying Toll NZ.
POT said container volumes exceeded 50,000 in November.
Toll and the port company have agreed to add 60 new rail wagons to the MetroPort Auckland service. The new wagons will be delivered in time for next year's peak cargo season.
Four trains a day travel between MetroPort Auckland and the port, with six trains on a Monday. At present, each train carries 94 twenty foot equivalent containers (TEUs) and the extra wagons will see this increase to 109 TEUs per train.
Weekly capacity will increase from 2700 TEUs currently to more than 3200 TEUs for next season.
Toll chief executive David Jackson said Toll had had a 16 per cent in the MetroPort Auckland business this year and had recently upgraded the service from three to four return trains per day.
At peak times deliveries were taking longer than customers wanted, he said.
Mr Jackson expects MetroPort's growth to continue.
"Despite speculation about ownership and the relationship between the Government and ourselves, it is business as usual for us and will continue that way."
There is speculation the Government is considering buying train operator Toll NZ.
Large rail customers and the state owned enterprise that owns the tracks – Ontrack – have been urging Finance Minister Michael Cullen to buy the company which is fully owned by Toll Holdings of Australia.
Toll and the Government have been involved in protracted and difficult talks about the access fee it should pay to use the tracks since 2003.
POT chief executive Mark Cairns said the establishment of MetroPort Auckland – essentially a cargo distribution centre in South Auckland – had proved a highly successful strategy that had resulted in a five-fold increase in POTs container throughput since it was established in 1999.
Over 150,000 TEUs were expected to be transported into and out of the greater Auckland region this financial year.