If spent properly, $16 billion will come back as tax income directly (by spent properly, I mean "if you have a bank account in Ireland, there's no need to apply for the funds, contractor). After contractor profits and material cost, probably $10-ish billion of that will go to guys actually doing work. Those people will no longer be unemployed, making a significant dent in the unemployment rate.
On top of than that, since this money goes largely to people without money, that money will get spent quickly, meaning products will be bought, businesses will be kept afloat by those sales, and those businesses will lay fewer people off by the truckload. Hopefully someone can convince them to spend it on things with a Made In America stamp.
The investment will likely mostly pay for itself when the lines are leased to private companies to run the lines after they're built.
The American people benefit by the additional infrastructure.
This is exactly how government should spend money. But obviously that's a huge amount of money and its application should be careful, thoughtful, and efficient. That's usually where these things go awry; they let private business tell them "what they need" instead of hiring an insanely over-qualified team to actually manage the job with Uncle Sam's interests in mind.