They don't stop at Stratford Mainline [SRA] station (where NXEA, LO, DLR & LUL stop) they have their own station called [stn]Stratford International[/stn] [SFA] which is part of the HS1 line.
I don't know the layout at SFA therefore cant answer your question further.
HTH
Edit: I have included LUL & DLR as these currently serve the same complex as the National Rail Services