Two trains will run on Saturdays and two on Sundays.
"Obviously the agreement does provide the opportunity to extend that in the future if it is going well."
Napier Port already has log-only weekday trains from Whanganui, Palmerston North and Woodville.
He said there were wider benefits as well. All the rail services took 1.1 million trucks off New Zealand roads, saved 200,000 tonnes of CO2 emissions, saved roading costs and saved lives from traffic accidents.
Napier Port chief executive Garth Cowie said Napier Port was a critical gateway for the central North Island "and ensuring that we have the right transport links in place is a crucial factor in moving East Coast export products to world markets".
The Wairoa link helped mitigate the large increase in forestry harvests, with the Port expecting a doubled log trade over several years.
"That growth cannot be accommodated by the roading alone, so the improvement in transport infrastructure - to get rail up and running again - is a critical component of that growth strategy."
He was confident the concept would be proven viable and the service extended.
Non-log freight was a possibility for the future but Mr Cowie said he thought that was unlikely unless goods were also going direct to Napier Port.
Mr Reidy was less enthusiastic about the prospect of extending the line north from Wairoa to Gisborne.
"The washout in 2012 was quite extensive. We are looking at various options for that part of the line and are open to options. No decision has been made yet - it is probably going to be a big infrastructure play to address the washout there but there are some tourism-related activities that might be of interest to us."
Napier Port is one of the largest international ports in central New Zealand. It processed more than 4 million tonnes of cargo in 2015 and volumes keep growing, necessitating a planned new wharf that can accommodate larger ships.