During World War Two millions of tons of material went east to Europe across the Atlantic; I presume that there was much less shipping the other way. So how were all those cargo ships utilized on the trip back to the US east coast? (I presume that the U-boats didn’t solve this problem for the Allies 
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Possibly raw materials?
All those American factories running at full blast must have been using lots of raw materials, that they had to get from somewhere.
In Britain, they were donating old pots & pans, pulling down wrought-iron fences, etc. to reuse the metal – could that have gone to the USA factories?
Raw materials? Northern Minnesota Iron Range and Appalachian coal fields.
War brides, dependants, casualties.
Casualties. Some, for that time, high tech electronic components that the Brits were geared up to manufacture and the US wasn’t. I’m thinking of magnetrons, maybe.
One little tidbit I’ve heard is that a section of FDR Drive on the east side of Manhattan was built on top of rubble from the Blitz that was hauled back as ballast in returning cargo ships that would otherwise have been empty.
My Maternal Grandfather showed me homes he’d build with brick carried back as ballast in some of those ships. They were maybe 10 years old at the time, mid 50’s, but looked much older because they were built with the old brick. Folks were willing to pay higher prices. These homes were build on Long Island.
And lots of Axis prisoners, especially later in the war.
This.
The troopships were used to carry POW’s westbound, but with many fewer on board (more troops heading in than POWs heading out). Hospital ships transported more passengers westbound than eastbound.
Mostly, the cargo ships heading west were in ballast. What cargo came westerly was a small fraction of that headed easterly.
I had relatives that traveled west from England to the US on the Queen Mary while it was outfitted for troop transport. The civilian passenger list was about 60, mostly businessmen and wives and other various folk.