West of England Mayor Dan Norris is taking a cautious view over the prospect of Bristol getting its own underground network.
The new Metro Mayor says he cannot rule the plans "in or out" because it is estimated to cost around £4billion.
Mayor of Bristol Marvin Rees outlined the plans in March, saying the mass transit system will improve the city.
The system could link Bristol, Bath and South Gloucestershire and North Somerset, and could be built in the next 10 to 15 years.
Marvin Rees says the network would help the city cope with a growing population and meet its environmental targets.
But new West of England Mayor Dan Norris appears unsure over the idea.
In an interview with BBC Radio Bristol, he said: "It's theoretically possible but I need to see all the plans. I understand the Department for Transport has been looking at the various options and we need to have a discussion.
"I will be a key player and I will do whatever needs to be done to get us a good public transport system.
"Whether that is an underground or not, I really don't know because my instinct is that undergrounds cost a lot of money because it is digging and money will be hard to get hold of.
"In the medium to short term, we need to do things with buses and in the longer term, consider exciting things like trams and other forms of major pubic mass transport. "
In April, Marvin Rees said that a shortlist of 19 options for a mass transit system had been sent to the Department of Transport.
Credit: Adam Postans, Local Democracy Reporter