Some 50 high-speed trains, including Frecciarossa services, were cancelled on Monday after fire on the line near Florence early on Monday morning.
On social media, pasengers said there was “chaos” at Florence's main station, where many passengers reported delays of four hours or more.
The fire affected services run by both Trenitalia and Italo in both directions on the main north-south line.
The fire, in a transformer room at Rovezzano on the outskirts of Florence, is believed to have been started by an anarchist group, local media reported.
Network company RFI said the fire was “a deliberate act by person or persons unknown”.
It said the average delay was 180 minutes and there would be repercussions for the rest of the day.
— Murs (@murs___) July 22, 2019
Regional rail services around Florence were also affected by delays, it said.
“Rail circulation was completely suspended from 05:00 to 08:00 this morning for checks on the part of judicial authorities”, RFI said.
“After the first checks were carried out, traffic resumed with strong delays”.
In Rome, delays of up to three hours were reported at the main Termini Station, Italy's biggest rail hub, whiile delays were also seen in Turin, Milan and other cities.
Milan Central Station, now. All trains southward 3, 4 or 5 hours late. Cause: arson in electric central near Florence. Lines closed. Police investigates … pic.twitter.com/vOkGieWI3z
— aart heering (@aartheering) July 22, 2019
Police said the arson attack may have been a “demonstrative action” by an anarchist group, with an oning trial in Florence against 28 anarchists for various offences against public order, the Ansa news agency reported.
Italian Interior Minister Matteo Salvini told Italian media that possible links to 'No TAV' protesters against a high-speed rail link from Turin to Lyon would be “verified”.