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There could be a few reasons for double heading Philip ,
More power, so the locos aren't working flat out.
Insurance if one breaks down.
Extra brake force to control the train.
Second loco could be on a balancing move , so rather than run it light loco and pay for track access charges , crew etc, attach it onto another train.
You are correct about class 90's double heading but as far as I recall it's only on the Great Eastern mainline.
The West Coast overheads are rated higher for " juice extraction" so double heading is the norm .
Cheers John
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