Are there plans to release new trains for AFOL?
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, accessories, 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.
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