The city has only acknowledged two "significant" disruptions in 2019, including the July 19th incident as well as one between March 21st and March 22nd. However, there there have been a host of smaller incidents, and delays can produce chain effects. The July 19th case led to 134 service changes on the No. 1 through No. 6 lines, and the glut of passengers switching to lettered lines created another set of delays.

The issue may stem in part from using older technology. Only the No. 7 and L lines have a modern Communications Based Train Control system that uses telecoms to improve positioning accuracy. They not only kept running, they're punctual over 90 percent of the time. Upgrades to CBTC are still ongoing, but they're expected to take years to complete. Locals may have to risk these kinds of delays for a while yet, and the city is learning a hard lesson about the risks of automating mass transit when the technology isn't bulletproof.