Complex sentence: In 1973 the prototype HST was evaulated during passenger operation in a saw 8 coaches formation between power cars, the two spare coaches were rebuilt and redeployed into service in the Royal Train (coaches 2903 & 2904) where they remain in use today.
Split into simpler sentences: In 1973 the prototype HST was evaluated during passenger operation in an 8-coach formation between power cars. The two spare coaches were rebuilt and redeployed into service in the Royal Train (coaches 2903 & 2904), where they remain in use today.