Class 20 locomotives on LUL

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Why are the S stock delivered between class 20 locomotives?

Is the delivery not a straightforward freight deal between a railway freight company and Bombardier to deliver S stock to Neasden?

I ask the question because the class 20's are very old locomotives and I wondered if more modern locomotives wouldn't be more economical and environmentally friendly.

[Deleted]

20's are already cleared for LUL metals, would be more expensive to clear another class

[Deleted]

There is the problem of a weak bridge between Harrow on the Hill and Neasden which precludes the heavier Class 66.

Class 66s have work engineering trains over parts of the Met and District since the late Metronet contracted GB Railfreight to work them.

Andy