As simple as it gets:

rule countHigh
when
$n : Number() from accumulate ( $sgc : StartGoalConn() count( $sgc ) )
then
System.out.println( "plain routes: " + $n );
end

And yet, it fails:

Exception in thread "main" org.drools.RuntimeDroolsException: java.lang.ClassCastException: rss.ixl.route.TrainStartGoalConn cannot be cast to java.io.Serializable

Sure, adding "implements Serializable" to StartGoalConn fixes this. But why is this necessary?

FYI, StartGoalConn is abstract superclass of TrainStartGoalConn - just in case this has some bearing on this issue.

Best
-W