An underground heavy rail line appears to have become the West Australian Government's preferred public transport option to service Perth's northern suburbs, sidelining previous light rail and bus proposals.

Premier Colin Barnett and Transport Minister Dean Nalder revealed a rail tunnel from Perth to Morley was being closely looked at, but construction would not start for many years.

Mr Barnett said the line could potentially be extended to Ellenbrook, an area the Government had earlier promised to expand the train network to before the plans were shelved.

The Perth to Morley rail proposal follows a 2013 election promise to build a light rail system, dubbed MAX, between the city and Mirrabooka.

That project was delayed after the election and Mr Nalder then spent time considering a rapid bus system instead.

However, Mr Nalder has now indicated an underground heavy rail line was his preferred option, and Mr Barnett said it was an idea that held strong merit.

"Probably many people living in those areas would like to see a heavy rail system, it will get them into and out of the city a lot quicker and in greater numbers," Mr Barnett said.

"Both options are there, and I would hope once the rail link to the airport is completed, that will be the next major transport project."

Mr Nalder said he was still exploring options, but fancied the heavy rail option.

"I want to find a better solution ... I admit that I favour the heavy rail and it is all to do with the capacity cost," he told 720 ABC Perth.

Barnett calls for time as Labor rejects plan

Mr Barnett said construction on whichever northern suburbs option was chosen would not start until the $2 billion Forrestfield-Airport Rail Line was built, with its construction due to finish in 2020.

He denied the Government's previous indecision and broken promises on northern suburbs transport options meant people could not trust them to implement the plans, saying the public understood they all needed to be carefully considered.

"You have to take time on these projects. They are huge billion-dollar investments and they need to be done in the right form," Mr Barnett said.

Labor was quick to dismiss the idea, saying it was nothing more than a thought bubble.

"They run on throwaway lines and what can get through the media cycle each day. We have seen that again and again," Opposition transport spokeswoman Rita Saffioti said.

"They have done nothing for eight years and our Government will have to go and clean the mess up."