The central bank's liquidity injections -- including almost half a trillion dollars earmarked to ensure New Year's Eve is a snooze -- and Treasury bill purchases have nudged the vital market for repurchase agreements back toward normalcy after a funding crunch sent rates soaring in September. This has anchored the Fed's benchmark rate firmly within policy makers' preferred range of 1.50% to 1.75% and caused T-bill yields to fall.

But next year will test whether the Fed can end its interventions without chaos re-emerging. Chairman Jerome Powell recently said the Fed isn't trying to eliminate all volatility from markets. However, if the repo market is erratic, it signals the Fed doesn't have good control over the financial system's plumbing. That's something policy makers and the broader market can't tolerate.

"It is all about credibility," said Peter Yi, senior vice president of short duration fixed income at Northern Trust Asset Management in Chicago. "Even if you announce some fancy new trains, you also have to make sure they run on time. The Fed is going to have an important role in how 2020 plays out."