While the "Buy American" idea was taking off under Nixon, the US Department of Transportation developed the "Standard Light Rail Vehicle" as a replacement for aging PCCs etc. across the US. Only Boston and San Francisco bought in, Philadelphia didn't because it wouldn't clear the City Hall loop. I never read specific reasons why Pittsburgh and Cleveland didn't. Wikipedia says you saw Boston #3401 in "mid-1976".
https://en.wikipedia.org/wiki/US_Standa ... il_Vehicle