I think the reference is to the North Low (Track 3 in passenger announcements). I haven't been to Trenton in a while, but as I recall there is no escalator to the low platform, and of course no level boarding. Additionally, if a last-minute track change is necessary involving Track 1 or 2 (aka the South High) it's no hassle for the passengers, who have only to cross to the other side of the platform; a last-minute change from either of those to the (so-called) Track 3 would probably delay the departure to later than it would be if the train simply waited for 1 or 2 to be available. I think the real problem is that Trenton Penn Station, like its big sister in New York, is handling more traffic, and at more concentrated times, than it was designed for. The original plans for the Metroliners envisioned spreading the tracks to provide an island platform between (actual) Tracks 2 and 3, but that never happened. (Imagine trying to do that under traffic.)