[rbarrois]

Indeed — when you look at this from other engineering disciplines: having your train track built for "no downtime, ever" means that you have a third track available, with its dedicated platforms, so that you can work on one track while traffic goes on.

This might make sense for your inner city loop, with trains passing by every 30 seconds (and it's gonna help when one train inevitably breaks down). However, it's a totally unreasonable cost for a station deep in the country where trains only stop 4 times a day.

[zahllos]

A maintenance window doesn't necessarily have to mean downtime either, it could just mean time you can make changes where people are on hand to fix it if things go wrong.

To extend your analogy, it might be a terrible idea to start doing maintenance on a single line track that sees high usage in a holiday season just before those holidays where everyone wants to get away or get home as do your staff. So you might implement what is usually called a change freeze.

On the other hand you might have a different time where demand is low. Shutting down the line if it comes to that will annoy some people but not as many. So you plan your maintenance then and have people ready in case things go wrong.

[drewcoo]

Adopting metaphorical analogs of all the impediments of real engineers does not make us real engineers.

Software is cheap to produce and quick to change. Building a "third track" on the fly, as needed, is exactly the kind of thing we can do that actual engineers can't. They probably wish they had the ability to do things like that.