The nearly weeklong stock-market shutdown that followed the 11 September 2001 attacks seems unimaginable in today’s world of round-the-clock trading.

During the past 20 years, advances in technology and broad efforts to bolster US market infrastructure have made such outages rare. Last year, when the coronavirus pandemic led to plunging stock prices, record volumes and an abrupt shift to remote work on Wall Street, the stock market stayed open and its core systems operated largely glitch-free.