The Adtranz IC-3 Flexliner and Siemens Regio Sprinter DMU's toured several parts of the US and Canada back in the late 90's. At one time, they were actually being seriously considered for Philadelphia to Harrisburg Keystone service but PennDot and Amtrak subsequently decided to renew the track and catenary in favor of electric power, a project that continues today. One of the problem with the European DMU's is that they lack FRA approval for crashworthiness with standard railroad equipment and thus their operations must be separated, either by dedicated ROW's or limited hours of operation. The NJT Riverline is a perfect example of this practice, with CSAO freight operations limited to overnight hours when the DMU's do not operate. One would have to assume that the demonstrators were all returned to their manufacturers.
One related note - the Colorado Railcar DMU's are FRA approved but have not yet been widely accepted.