Portland regularly mixes their Bombardier and Siemens cars and seems to have no problems. Since the Portland system is low level boarding each Bombardier car (which is a high floor) must be accompanied by a low floor Siemens car.

The Skoda cars on the Portland street car line are never paired with any of the Tri-Met light rail cars since they are seperate systems and were never intended to share equipment. There is a connecting track between the two systems but that is only to permit a street car to access the wheel machine at Tri-Met's Ruby Junction maintenance facility.