Great question...and possibly not an easy clear answer. Cuba's trains have generally been in poor shape in recent years as older equipment has run down and spares not available.
However, according to Mark Smith (The Man in Seat 61), that's changing with the arrival of a fleet of new Chinese-built locomotives. On his site (Seat61.com) he has an extensive update of recent changes in schedules, routes and locations (that's important because Havana's main station is closed for a 3-year rebuild).
He does warn to check on the spot: current information changes frequently. If you go, by all means, keep us posted here or with blogs and pictures!