Hi Fastpedaler. Welcome to the forum.
Realtime trains shows a northbound Wembley to Barrington Unloading Pad spoil train passing Stevenage at 11:02. It also shows a southbound working from Barrington to Wembley at 13:25.
Could these be the trains that you see?
If so then the afternoon working is not actually the empties of the one you see in the morning as it leaves Barrington at 11:23.
HTH
Incidentally Realtime trains says this mornings working is about 15 mins early.
