Are you referring to the Washington County (Oregon) service, with 3 powered cars and one trailer?
Yes - the reason for having three powered cars is that if one is in 92 day insection (per FRA requirements, because it's considered a locomotive) that another car is available.
As for "easily replacable", not sure that's possible. At most the trains would be two cars (either powered-powered or powered-trailer; the original plan was to buy two trailers and three motors, but TriMet conveniently ran out of money.) There aren't any passenger cars around to substitute; Amtrak equipment couldn't fill in, and TriMet's insistence on high platforms (instead of low-floor equipment) negates the ability to lease equipment from Sound Transit if needed. Nor are there spare locomotives easily acquirable; and the station platforms will only be designed to accomodate two-car trains; so a locomotive hauled trainset is out of the question unless the train is only one car plus locomotive (it couldn't be two cars, because of the location of station platforms and adjacent streets at least one car would be out of reach of a platform at both Tualatin and Tigard, and possibly Washington Square as well. And I'm not sure a three-unit train would fit at Beaverton.)
The only logical "substitute" equipment would be a single Amtrak locomotive and either an Amfleet-I or Horizon car, and those things just aren't exactly plentiful around here. Or find a passenger coach from one of the historical societies, which aren't ADA compliant and don't have functioning HVAC systems...and probably don't have legal emergency exits either.
Erik Halstead - Portland, Oregon