The State Government believes the MAX light rail system will generate about 100,000 passenger trips a day by 2031, compared with 29,000 trips a day for the airport-Forrestfield link and 10,000 a day for the Ellenbrook link.
But it is not as simple as comparing the numbers, Transport Minister Dean Nalder says.
"Boarding projections for new stations such as Butler, Yanchep or Aubin Grove cannot be compared to projected passenger trips along a new service like MAX, Forrestfield-Airport Link and Ellenbrook," he said.
Another complication when comparing projects is substitution effects on different projects.
Thousands of people use bus services in the northern Mirrabooka corridor, and east-west from Nedlands to Victoria Park. MAX would also likely replace some free CAT bus services in the inner CBD areas.
"Public transport planning takes into account more than a return on investment and passenger trips," Mr Nalder said. "It involves weighing up the social and economic benefits of any given plan to make sure (people) have equitable and appropriate access to infrastructure, regardless of where they live."
The Opposition is sceptical of the Government's modelling for the Ellenbrook link versus the airport-Forrestfield link.
"The Government claimed there were only 60,000 people in the entire catchment for the Ellenbrook rail line but if we go to the City of Swan forecasts, more than 100,000 people will live in the catchment," Labor MLA Rita Saffioti told Parliament last week.