We've taken our tandem on trains in Europe many times. In 2014 we took it on a train from Ancora, Italy to Rome and then later, from Rome to Florence Italy. No problems really. Once in Czechia (Czech Republic) the conductor told me the tandem was too big for the "bike car" and we put it in the cargo car (they allowed me to put it in and tie it off). As Merlinextralight said, use local and regional trains. To my knowledge no bikes are allowed on the high speed trains in Europe.
Tailwinds,
Charlie
2016 Santana Beyond