I just built the Maersk train, what an amazing model! I did a little research and found that it was released in 2011 and the Emerald Night was released in 2009. Given the two year gap it seems reasonable to assume there will be new train in 2013. Does Lego have a schedule or patterns for this kind of thing?
With the exception of evergreen themes like CITY and Duplo, it's not easy to predict when sets are released. Based on trends of the last 11 years, I've included the last 20 train sets that have been released in a table:
At least one train set has been released each year apart from the year 2008. The table marks an average of 1.66 train sets per year. It's fair to assume that a new train set will be released in 2012 or 2013.
Duplo, accecories, motors and track sets not included. I've included the Western Train Chase from the Toy Story theme, The Constitution Train Chase from The Lone Ranger theme and The Ghost Train from the Monster Fighters theme which you may wish to exclude as anomalies. The right-most column marks which sets were sold with track.
Set data gathered from Brickset.
Check the new Creator "Advanced" set : 10233 Horizon Express
The Red Cargo Train is the most recent train set released after the Maersk Train