These were considered powerful enough to stop a heavily loaded bike going downhill.
The sidepull brakes of the time were considered less capable for such extreme duty.
It wasn't until Shimano developed their "V-brake" (known generically as the side-pull cantilever or the direct pull brake) that a viable alternative to the traditional cantilever was available. The table below shows these two 520 characteristics through time.
These data should be of use to people who plan to buy a vintage 520.
The long chainstays, 47cm, provide heel clearance for the rear panniers, and contribute to a comfortable ride.
These frames and bikes are still highly sought after by knowledgeable tourists.
The steel racing Treks will never again be used by top racers for racing.