An obvious example is the Gold Coast Railway, it was such a big mistake that they ended up having to rebuild it. The rebuilt line might be a bit straighter and therefore quicker, but I reckon the Southport connection still would have been good. You would have been able to walk straight off the train and onto the beach.
Melbourne's Webb Dock line had the potential. Unfortunately the line was never given a proper chance to prove it's worth.
I reckon Melbourne's outer circle line would have been handy if it was still around, it would have given Metro ways to route around failures and accidents.
In future years I believe they will regret closing the Murwillumbah line. It would have been quite easy to extend it towards somewhere like Kingscliff where the Gold Coast population is slowly moving more and more south (just keep it going alongside Tweed Valley Way). If they extended the Gold Coast tram to connect with it you would then have a perfect transport system.