Although stone trains for Acton will take the new line from Reading West to the Reliefs under the flyover it seems trains will not be able to wait on this line due to possible problems restating such heavy trains on the gradient.
Therefore they will have to be given a clear run from Theale.
Is the gradient a problem? This has been discussed here before and I seem to remember that other posters didn't consider the gradient to be such an issue. Essentially the Up Feeder line from Oxford Road junction goes under the flyover at approximately the same level as the previous lower yard. The embankment carrying the Main Line is about 3 metres high hereabouts so the line has to rise this amount to join the Reliefs over a distance of about 500 metres before the Caversham Road bridge, This gives a gradient of 1 in 166 or thereabouts, within the capability of a Class 59 on a 4,000 tonne train. The acceleration might be a bit slow though! On the other hand the rear of the train will be on the flat or on the falling gradient from Oxford Road to the flyover.
The length of the train might be a bit critical if the positioning of the points and signals means that halting a full length train might block a junction. However
NR» is working to a standard freight train length of 775 metres so I would hope that a train of this length could be held on the feeder line clear of the junctions at each end. If this is not possible then, as you say, they will need a clear run.
We'll see!