Throughout the end of the 1800s and the first decades of the 1900s, passenger traffic steadily increased. The coaches being gradually built or incorporated through FNM’s acquisition of lines previously operated by other companies largely maintained the original characteristics. Although significantly increasing the number of seats or the width of the modules, those subsequently supplied by Italian or foreign companies including Maschinenfabrik, Diatto, OM, Reggiane, Carminati & Toselli, Grondona & Comi and Costamasnaga were still of the two-axle type with access platforms at each end. The last third-class coaches built with these characteristics were delivered in 1927, when the first first-class bogie coaches were already being supplied.
The Az first-class bogie coaches, delivered from 1923 on, were the first coaches fitted with bogies and toilets to circulate on the FNM network. This improved the comfort of the first-class service previously provided by the two-axle coaches. 6 new bogie coaches with a brand-new all-metal design body, unlike the previous all-wood versions, were delivered for the third-class service in 1931. Some of the first-class coaches were also converted into third class, due to the continued increase of commuter traffic.
With regard to the locomotives, additional, more powerful locomotives were added to the original 6 Couillets as early as the beginning of 1879. Other locomotives, also of a small size and modest power, were added to FNM's rolling stock through the acquisition of lines operated by other companies, which were gradually taken over by FNM along with all the rolling stock in service on those lines. This was the case for the class 71-77 locomotives, later renumbered as 270-01-07. The first two in the series were inherited with the acquisition of the Novara-Saronno-Seregno line, previously operated by the company of the same name. Their performance was so impressive that five more identical ones were built. One of these, the 270-04, still exists today.
In 1894, in order to improve the direct train service, the class 251-262 locomotives were commissioned, the first to be capable of speeds of 80 km/h.
In the early 1900s, goods traffic had now increased so much that not only were new wagons required, but also more suitable locomotives. Such as class 401-411, and later class 240. One working example of these still survives, the 240-05.
In the following years, the steam traction fleet was further expanded with the delivery of class 220 locomotives for goods trains and class 280 for passenger trains. These last were FNM’s most numerous and well-known class with 975 hp and a maximum speed of 80 km/h, and a load capacity of 650 tons.
Finally, in 1931, the 4 class 290 locomotives for direct trains, the last and most powerful of FNM’s tank engines, went into circulation. These were magnificent machines of 1100 hp and a 90 km/h maximum speed, which nonetheless were relatively short-lived as electrification was already underway.