It is unavoidably dangerous, and planners are supposed to choose it only when other options aren't practical, but in reality they often pick "Open line" for convenience. For example maybe the workers would prefer to do this on Tuesday midday, so we plan it as "Open line" and schedule it for Tuesday 1145, rather than for 0400 on Thursday when we can just shut the line to do it safely. This gets people killed, but because it doesn't reliably kill the specific people you scheduled most of the time it feels OK to them and to you.
This is also why unions aren't enough to implement safety. Unions represent workers and the workers feel like convenience is more valuable than safety. A 1-in-1000 chance to die, for five minutes extra coffee break is a good deal, until you're the 1-in-1000 and then it sucks. In the UK signaller unions fought very hard to continue working 12 hour shifts instead of maximum 8 hours. Because working 3x 12 hour shifts and then the rest of the week off is _convenient_ even though it was excessively dangerous if anything goes wrong.
One of them has a lookout at each end, who alerts the workers whenever a train approaches. Work stops until the train has passed, because the distraction caused by operating loud and heavy machinery is ao high. (The train only sounds the horn once, unless the driver fails to see an acknowledgement from the lookout.)
However, this isn't really practical on a busy line like a metro, so they'd much prefer to work overnight with the line closed if possible.
It's also possible to fence off the work area from live tracks, but installing the fence is then the disruptive work.
> as long as these workers are smart enough to not encroach on the active lines.
That attitude is completely at odds with the European approach to worker safety.
(I cannot comment on whether either point applies to the USA.)
As an example, there's are descriptions of the failures of the safe working system in the reports of accidents, such as this one: https://www.gov.uk/government/news/report-122019-near-miss-w...