The state government considered bolting on a carriageway beneath the Sydney Harbour Bridge or building a viaduct above two lanes of traffic for a new metro rail line but eventually decided tunnelling under the harbour was the best option.

In laying out the justification for the second stage of the $20 billion-plus rail project, a truncated version of its business case reveals that a suspended carriageway or a viaduct would have cost at least $400 million more than tunnelling under the harbour.

The two options for running a new metro line on the bridge also had a "number of constraints", such as posing a barrier to ships passing beneath or detracting from one of Australia's most famous landmarks.

"These options would also have broader network impacts during construction and operation (particularly in terms of access to the Sydney CBD for other transport modes). Consequently, these options were not progressed further," it said.