Exmouth Junction to Topsham was singled around 1973? Because the new M5 was being built underneath it, a new rail bridge was required. Who ever paid for it (BR, council, highways?) Didn't want to pay for a double track bridge, so single line one was built. Which is fine except you now have a dead end siding with one passing loop, no spare capacity for recovery ( delays loading rugby fans on match days) or any chance of changing or adding services as the line is at"capacity" some of the time.
An "urban myth I have heard ( or should that be rural ? Is that Plymouth - Exeter via Okehampton ( double track " mainline") was supposed to be singled instead of tearing up the Bere Alston/ Meldon section as a useful diversionary for the sea wall. However the contractors miss understood and ripped out both down and up lines. By the time BR realised it was too late as too much damage had been done. After it was closed,but before it was ripped up BR did run some freights this way when there was a problem with the sea wall .(Even though they had already removed all the signalling and telephone lines) - which lends weight to the theory. Although I'm very doubtful about it.
A line singled, Carrys double the traffic of the double track, so x2 wear and tear? Increasing costs? Unless like beeching you drastically reduce the traffic to make the singled line have no more traffic than either of the up and down main line when it was double track?