Complex sentence: Several plans had been proposed over the preceding decades to alleviate the bottleneck, but none were to see the light of day, except for the concept of a circular railway allowing trains to continue past Flinders Street, turn around and return to the suburbs.
Split into simpler sentences: Several plans had been proposed over the preceding decades to alleviate the bottleneck, but none were to see the light of day. That is, except for the concept of a circular railway allowing trains to continue past Flinders Street, turn around and return to the suburbs.