Thanks Beags for the graphics. When you turn off the A41 and drive through the village of Westcott (yellow road on the OS Map) you can see the entrance to the airfield/industrial sight off to the right along a straight section of road - this is clearly the old tramway. It also explains the brick railway abutments on the A41 about 1 km towards Bicester - obviously the remains of a long-removed railway bridge on what your old map tells us was a GCR line. The old map also shows a tramway station called Waddesdon Station a short distance beyond the road junction towards Aylesbury although the village of Waddesdon is some distance to the west.
I know, I know, I probably need to get out more.