TRANSPORTATION SECRETARY STEPHANIE POLLACK said on Monday that she saw no inconsistency in pushing ahead with a $2 billion plan to expand South Station even as the state is spending $2 million to study the feasibility of building an underground rail link between North and South Stations.

Backers of the so-called North-South Rail Link, including US Rep. Seth Moulton and former governor Michael Dukakis, say it would make no sense to both expand South Station to accommodate more commuter rail and Amtrak trains and build the North-South Rail Link. With the rail link built, they say, trains could be stored overnight at numerous track locations and wouldn’t have to occupy very expensive real estate at South Station.

Moulton could not be reached for comment on Monday, but a slide deck on his website says “South Station expansion threatens development and is costly and unnecessary.” The slide deck says the cost of the South Station expansion would be $2 billion, compared to $3 billion to $5 billion for the North-South Rail Link.  The document indicates the South Station expansion would prevent $10 billion in downtown Boston development.