The Birmingham Direct line, between Old Oak West Junction and Northolt
Junction, is still used occassionally for freight workings,
parliamentary trains, ECS and diverted passenger trains. When the link
line from Neasden to Northolt is closed and Chiltern trains cannot
reach Marylebone, the Direct line is used to run Chiltern services to
Birmingham from Paddington.

The line is single between Northolt Junction and Greenford West
Junction, double thence to Park Royal West Junction and single to Old
Oak West Junction. Linespeeds are appalling along this stretch, with 40
being the average for a route that was once 90.

When and why was it singled? I travelled on the West Ruislip branch of
the Central line sometime around 1990, and vaguely seem to remember
the Direct line being all double track then (though I might be wrong).
What reason could there have been since then to go out and single it?