In the 90s, ADTranz (now Bombardier) recreated a 4Cig unit in a Electrostar-style body, keeping the controls and electrics the exact. It would've costed 3/4 of the price from a new-build and estimated a life time of 15 more years. This was called the 'Networker Classic' project, also known as the Class 424. Unfortunately the newly-privatised companies ordered new-builds so the idea never really took off.
If I were SWR, or a new TOC, I'd rebuilt some Class 455/456s and create them into five car trains, using the same method what ADTranz did.
I know there's a topic on this, but it's too old to discuss more on it.
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