I'm guessing this was the early running 6M28 Hinksey VQ to Bescot Departmental.
It has been put inside the loop to allow another train to pass or to await its booked path. The driver opens up the power handle to keep the train moving on the rising curved gradient.
This train will overtake 66219 in the loop, 168217 and 168110 form 1D51 1741 Marylebone to Stratford upon Avon.
