So after a ship made it's delivery to Europe, I am assuming most then made a return trip to the US. I have a couple of questions regarding this.
What if anything did they carry on the way back to the US? Seems like it would be a waste to send them back empty, but I also can't imagine what might be sent back? Were there enough wounded soldiers and PoWs to fill them up? Did they bring back natural resources or other goods from the countries they delivered to?
On average how many times would a single troop transport or cargo vessel make the Atlantic crossing during the war?
Not a complete answer but an interesting one. A small part of Manhattan (on the East River) is actually made up of literal English soil and houses that transport ships brought back from Bristol. They needed some ballast to stabilize the ships so they loaded them with rubble. The area where this was dumped is now called Bristol Basin. http://www.atlasobscura.com/places/bristol-basin this provides a link to the plaque commemorating this.