This will help cut energy costs... Old news, but new technology still

Then I hit google ...

Keeping telephone cables dry, and displacing arms and legs ...

In the UK, telephone exchanges contain units which dry and compress air, and this is pumped into the cable jackets where they leave the exchange. Many of the old very-many multi-pair cables have what looks like paper insulation, but is more accurately described as "air insulated, paper spaced" insulation. It's clearly essential this stays dry, and these cables were designed from the outset to be run with pressurised dry air passing through them.

I was in a small village exchange about 10 years ago, talking with one of the exchange engineers, and the air compressor was starting up periodically to charge up the pressure vessel. He said the management decided to switch it off at one point. Within a month, they were inundated with faulty lines, and it took 2 years of running the compressor to get back to the previous state of the lines.

I suspect such cable is not used in new installs for some time now, but there's loads of it in use in the ground.

Not just phone cables

The Forth Road Bridge is having dehumidification fitted to stop corrosion in the [corroding] support cables, nevertheless its service life could be just a few years.

OTOH The Oresund Bridge had this from the outset and should last centuries.

You have to wonder if those tanks give off enough nitrogen to create a suffocation hazard in the cable vaults or even leaking into subway stations.

I've always seen active ventilation used when they're working in cable vaults. Subway stations are pretty big and I would hope there is some active ventilation there too. They'd get pretty rank otherwise.

What, like big pistons (a.k.a. trains) running in tubes (tunnels)? Certainly, the London Underground is very windy. (I haven't used the New York subway for about 13 years, and I can't remember.)

No, like ventilation shafts and fans. (tunnels are too long).