While the idea behind the North South Rail Link has been around for decades, there are at least two big reasons advocates of the rail-link project are stepping up their push in recent years.

One is the new development of tunnel-boring technology, which reportedly allows for tunnels to be built underground without disturbing the ground above. Proponents argue the notorious cost overruns of the Big Dig were mostly due to the costs of preserving historic artifacts and rerouting street traffic during…