problem in sorting when using subtable

[ashutoshdeora]

i am putting my code
i am using subtable but i am not able to sort
please can anyone tell me if i am missing something in the code or i am doing somethign wrong

<rich:dataTable value="#{fList.trainResultList}" var="rList"
 width="100%" rowClasses="odd,even">
 <f:facet name="header">
 <rich:columnGroup>
 <rich:column sortBy="#{dList.trainName}">
 <h:outputText value="Select" />
 </rich:column>
 <rich:column sortBy="#{dList.origin}">
 <h:outputText value="Train Name" />
 </rich:column>
 <rich:column sortBy="#{dList.destination}">
 <h:outputText value="Origin" />
 </rich:column>
 </rich:columnGroup>
 </f:facet>

 <rich:column colspan="8">

 <rich:subTable value="#{rList.trainDetailList}" var="dList">

 <rich:column >
 <h:outputText value="#{dList.trainName}" />
 </rich:column>
 <rich:column >
 <h:outputText value="#{dList.origin}" />
 </rich:column>

 <rich:column>
 <h:outputText value="#{dList.destination}" />
 </rich:column>

 </rich:subTable>
 </rich:column>
 </rich:dataTable>


 </rich:column>
 </rich:dataTable>

this is the part of my code where i am using the subTable
please can anyone tell me weather sortBy can be used or not
if Yes
then where i am making mistake or missing something
or i have to do something else