it'd be a bus connecting at Truro. It was sad to see the tracks rusted away through Cape Breton on our visit in 2017.I was tempted to also suggest Moncton-Halifax, but with the new expressway, and winding railway track, I just don't see how buses aren't more effective.
Is Sydney even an option any more? I thought that there wasn't any service other than just across the causeway. Globe and Mail says it was abandonded in 2015 -https://www.theglobeandmail.com/report-on-business/railway-shuts-down-cape-breton-line-after-135-years/article22472332/
As for the Newcastle Sub, it's a lot more scenic than the Edmundston Sub as it follows the coast. I'd be sad to see it abandoned, even thought it'd mean a faster trip. The interior of New Brunswick is incredibly boring, like Northern Ontario but without all the interesting rock formations.