The Tory leadership front-runner made the comments as he visited the construction site at Manchester Airport's new Terminal 2 extension, which is expected to open next year.
Bosses at the airport have made it clear to Government they want both Northern Powerhouse Rail (NPR) and HS2 to happen and provide a "global gateway" for the north with quicker and more frequent rail journeys to the airport.
On Tuesday, Mr Johnson was enthusiastic about the case for NPR as Transport for the North chief executive Barry White and Manchester Airport group chief consecutive Charlie Cornish outlined its benefits.
He later told reporters: "This is going to be the heart of Northern Powerhouse Rail and they are making a very, very good case here for GBP39 billion investment in east-west connections from Liverpool to Manchester, Bradford, Leeds.
"A really high speed line that will bring together those northern cities, bringing loads of people - about eight million people - within commuting distance of Manchester Airport so you have suddenly have a real expansion of this airport's capacity to deal with the UK's needs.
"It is a very, very exciting thing. Don't forget if you put money into a transport infrastructure you create opportunity for people on modest incomes around the country.
"Mass transit great rail projects equalise society. They give people an opportunity."
Mr Johnson appeared more cautious on the merits of HS2 and said there was "a legitimate question to ask about how the money is being spent".
He said: "Just in my own constituency in Uxbridge they suddenly discovered that they did not have a sufficiently long extension lead for the tunnel boring machines.
"They had to spend GBP20 million digging under the road. "(So) is the money being wisely spent? Could we spend it better?
"But as I have said many times I am a big, big passionate supporter of great public infrastructure, I think it's vital for our country."
Asked whether his stance had anything to do with HS2 running through his constituency, he said: "The awful truth is HS2 certainly doesn't do my constituency any particular favours but I have resolutely refused to say I would cancel HS2 because, as I say, I hesitate before doing things that would stop great public infrastructure projects.
"But what we need to look at is the size of the bill, it's probably north of GBP100 billion by the time it's done.
"Is there anything we can do with the profiling of that money, is there anything we can do to accelerate the east-west bits in the north while not delaying the spend on the London to Birmingham line?
"That's the issue. But we will not be holding up the programme. I just want to satisfy myself that it's really being properly spent."
Mr Johnson asked for a review of the HS2 scheme by its former chairman Douglas Oakervee, which is due to be completed by December.
The extended departure terminal at Manchester Airport will more than double in size as part of the airport's GBP1 billion transformation.
The first phase of the programme opened in April with the launch of a new passenger pier and a 3,800-space multi storey car park.
When the terminal extension is complete and opened the existing Terminal 2 building will close for refurbishment before it will be fully operational by 2022.
Boris Johnson has praised plans for Northern Powerhouse Rail (NPR) as "very, very exciting" but maintained the costs of the HS2 project need to be looked at.