Viazul (http://www.viazul.com/) buses have a twice daily service from Havana stopping at Cienfuegos, and a daily service from Santiago de Cuba stopping at Holguin, Camagüey, and Sancti Spiritus. There is also a service to/from Varadero stopping at Santa Clara and Cienfuegos that is not listed on the Viazul website. It leaves Varadero at 7:30AM and returns from Trinidad at 2:25PM. Viazul and Cubanacan offer direct buses to/from Viñales as well. You buy Viazul tickets from the air conditioned office right in front of where the buses park, not from the Astro or local bus counters.
Travel via taxi, specifically, unofficial taxis are a great cost effective way to travel to Trinidad as well. Typically only costing double the bus fare for two people, you'll get there in less than half the time, and in much greater comfort. When traveling with children, you might find this the preferred method of travel.
Trinidad has not been connected to the rest of Cuba by train since a hurricane destroyed an essential bridge in 1992. There is a local train running to the Valle de los Ingenios.
