The Viaduc de Morlaix is the defining structure of the town — a double-decker railway viaduct that strides across the valley on two tiers of granite arches, its highest point sitting 58 metres above the river. You see it the moment you arrive, from the train (you're on it), from the town below (looking up), or from the surrounding hills (looking across). It gives Morlaix a grandeur that the town's modest size doesn't otherwise suggest.
The viaduct still carries the Paris–Brest TGV line, so trains pass over it regularly — worth timing your wandering to catch one rumbling overhead. The best views are from the old town below, where the scale becomes most apparent. You can also look down from the top if you're arriving by train; the view over the slate rooftops is worth getting a window seat for.
No special access is needed to view the viaduct — it's simply visible from much of the town. The area beneath it has been partly pedestrianised and is the informal centre of the old quarter.