Well if you play your cards right, and build the sets right, it is possible to make the sets easily expandable. Right now the Talgos in Cascades service have end cars that are baggage on one end and power on the other. Then on the end of these cars you have standard North American couplers. In theory, if you either pull your HEP from the engine, or put both service cars on one end you could easily add cars. If the tilting mechanism is turned off you could add on Horizons/Amfleets for crush capacity times. Not likely to happen, but it seems plausible.
Also, don't forget, in busy times (like Thanksgiving) you may be better to follow the Northeast model and add extra trains offering more departures than to expand capacity on one particular train.