The original line, the hilly Porrettana via Pistoia, is still there but it's slow and you need to change at Prato.
Nowadays you have to change at Porretta Terme and Pistoia, but it's very scenic -especially the Southern section between Porretta and Pistoia - and well worth doing if you have the time. Porretta itself is a pleasant and relaxed little spa town high in the Appennines with several reasonably-priced hotels and B&Bs, which would make a nice overnight stop if you are in the area.
The other very scenic trans-Appennine route in the same general area - but, unlike the Porrettana, not electrified - is the 'Faentina' from Firenze to Faenza, with connections from there to Ravenna. This line has two branches from Firenze as far as Borgo San Lorenzo at its Western end. The through trains to Faenza via Vaglia - approximately every two hours - are formed of 'Minuetto' 3 car articulated DMUs which, although slightly noisy and rattly nowadays (built 2005-07), have big windows which line up with seats in the two end cars and are great for enjoying the spectacular views in the mountains. The other route, which terminates at Borgo SL, serves a more populous area - although still with very pleasant hilly Tuscan scenery - and runs hourly with a mix of 'Minuetto' DMUs and class D445 diesel loco powered push-pull sets.
If you like 'Private' lines - actually nowadays owned by a consortium of different local authorities - the Trasporto Ferroviario Toscano line from Arezzo to Stia is a very pretty run up the Arno Valley between the Pratomagno mountains and the main Appennine chain. Pre-Covid, it - and its sister line from Arezzo to Sinalunga - were operated by a veritable working museum of ancient electric locos, railcars, motor luggage vans and carriages. However they are all now stored in various states of disrepair, vandalism and graffiti in sidings at the various stations along the two lines and services are operated by modern 'Elfo', 'Jazz' and 'Pop' EMUs identical to their Trenitalia sisters. Incidentally, you buy your tickets for these lines from the bookstall in Arezzo station and then stamp them in the green and grey Trenitalia validators.
A final scenic route in Tuscany is the 'Garfagnana' line from Lucca to Aulla Lunigiana, which follows the valley of the River Serchio between high, rugged mountains before passing over high viaducts and through long tunnels, then descending to join the La Spezia-Parma line - itself quite scenic - at Aulla. This line is now operated entirely by Polish-built Pesa class ATR200 'Swing' three car DMUs and the best views are generally on the West side of the line....except between Borgo a Mozzano and Bagni di Lucca, were a change of sides is required to see and photograph the incredibly-graceful 11th century bridge across the river, 'Il Ponte della Maddelena', one end of which the railway passes underneath.
