The only example of a national railway museum that I knew of before visiting this one is the York Museum.
I believe that Pietrarsa can hold a candle to York in terms of the richness and value of the material on display. In the meantime, its location in the former factories created in 1840 by the Kingdom of Naples, for maintenance and partly also for the construction of railway material, is remarkable.
The historians of modern Italy are well acquainted with the tale of Italian railways starting from 1839 (date of the opening of the first section, Naples-Portici, which still passes just outside the Museum). For instance, they note that this early departure of the railways in the Kingdom of Naples doesn't correspond to an equal development in the following twenty years (those preceding the Italian unification): in 1860 they were operational in the Kingdom of Naples (southern Italy plus Sicily, by far the largest state in Italy) 127 Kms of railway, compared to about 100 Kms in the Papal State, 332 in the Grand Duchy of Tuscany, 656 in the Kingdom of Lombardy-Venetia (Austria) and 835 in the Kingdom of Piedmont-Sardinia.
The grandeur of these industrial "containers", now used as a museum, is all the more astonishing. In particular, the pavilion hosting the steam locomotives (and also some electric ones), at the southern end of the museum area, is impressive; both because it's the largest of all (a real "cathedral"), and because it contains a very rich series of steam locomotives, each adequately illustrated with captions in Italian and English.
But the other pavilions are also noteworthy; such as the one that, in addition to many electric locomotives, houses, among other things, a coach from the train of the King of Italy from the 1930s, and a rare coach for the transport of prisoners.
Yet another pavilion has an assortment of diesel-powered locomotives; another retains the original machinery for the maintenance of the locomotives; finally another one preserves models of trains, including two large working models: one reproducing the Florence station and the line to Bologna and another reproducing the Rhaetische Bahn line from Tirano (Italy) to St. Moritz (Switzerland) in its initial stretch, the one straddling the border.
The location of the Museum has no equal in the world: from the external terraces the view sweeps across the entire Gulf of Naples, including the islands of Capri and Ischia, while Vesuvius looms behind it.
It's probably possible - I write this always bearing in mind the example of York - to do more in the sense of the museum's interactivity (British institutions seem to me to be masters in this). But already in this way the Museum is an outstanding - and quite unexpected - attraction of the Naples area.