Well, the simple answer is it can't. For starters, the existing corridor is vastly unsuitable for any sort of high speed service, as it was planned about 100 years ago, too many sharp curves.
So a new corridor would be needed, requiring significant land acquisition in many locations.
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The other aspect is safety. Rail Trails are very popular with families, including small children on 'kickalong' bicycles, because they are completely safe.
Can you imagine any scenario where a child could ride on a path that has a train passing nearby travelling at 100kph?
In some sections the existing trail is only six metres wide. Imagine the amount of land required to safely separate a child cyclist and a high speed train.
So yes, a nice idea but unfortunately rather impossible.