@Running-Away Per page 22 of the rulebook, units of another power may be transported only if that power is friendly. Per page 12, the US is not considered to be either friend or enemy to any other power while it remains neutral. Therefore, the US cannot transport units belonging to any other power while it is neutral.
One thing I like to do as well is to have a transport or two in sea zone 8, then ferry forces from the US directly to Ireland (it only takes 1 turn for ships to move from there), then onwards to Picardy or Brest (the transports in sea zone 8 bridging). They can also move into sea zone 9 and desposit them in Belgium depending on the situation.