It asserts that by 2030, a rail system could pump 28,000 tons of harmful pollutants into the air.
"This rail will be soot black. Definitely," traffic engineer Panos Prevedouros said.
But city managing director Kirk Caldwell said the findings are flawed and don't factor in automobiles.
"We're taking 30,000 cars off the road and they somehow say that that's not going to help on greenhouse gas emissions? It's crazy," he said.
Small Business Hawaii paid mainland company Wendell Cox Consultancy $5,000 for the study. The company advertises itself as an international firm that specializes in urban policy and transportation.
Prevedouros said Wendell Cox is a top environmental transit analyst who used Environmental Protection Agency models for his findings.
A memo from the city called Cox a "hired gun" for the automotive and oil industries.
"For him it's all about building more roads and more highways," Caldwell said.
The study does concede that rail could control pollution during rush hour but not during off hours.
"Eighteen hours of the day it is actually a heavy, heavy polluter because there is no rider-ship," Prevedouros said.
In defense of rail Caldwell points to the Draft Environmental Impact Statement.
"If they use the numbers that we provided they would see that it does in fact result in a reduction of greenhouse gas emissions," he said.