This would fail at the first hurdle.

1. Planes can determine their altitude. Trains don't know which track they're on. You'd have to re-engineer existing signalling systems designed around the inverse. Track transponders and axle counters know which trainset passed a specific piece of track, but the trains don't know which track transponders link together on the same track. They don't even know which track they'll be going down next because that's up to the signallers controlling the points, not them.

2. Planes fly in the air and can send omnidirectional radio signals with little to no interference. Trains run on the ground, often in valleys and tunnels. There is no guarantee that trains broadcasting "I am here" signals can be picked up by the next train within the stopping distance.

Many trains did broadcast "I am here" signals, but once speeds got faster, the broadcast range (about 1km) wasn't far enough for a train to stop in time before a collision, which is why trains need to communicate, using on-track wires that run the length of the track, to each other and central control. Which therefore means that the central control system needs to work, and when it doesn't, the trains have to stop.

Planes use radio signals because they don't have signalling cable following their entire route. Trains do, and it's usually much more reliable than radio.

Except, trains already have them since about 150 years ago. The problem was that the "train TCAS" requires minimum distances for detection, so to speak, below which the system can't detect anything other than itself. When the newer system went down, there were more than one train within that minimum distance, so the older backup system didn't detect anything other than itself.

They can upgrade the system so that the minimum distances are smaller. Sure. Except, such hypothetical upgraded system was already in place, since 90s, and __that was what went down__.

If you were to then ask me, okay, why did they get the trains apart before committing with the downgrade into the older system. And that would be the right question. It's such a braindead move to not do that before executing the full downgrade. It's not like trains were moving when downgrade took place and it couldn't be done, the newer system going down triggered emergency stop of all trains. Train systems are intentionally built that way. So they could (load-bearing-)just ordered the trains to get spread out so there will be enough distances between trains so the older version of "train TCAS" would work correctly. The decision to NOT do that, with hindsight, is downright reckless.

By the way, if all reports on this matter is to be believed, the required distance of the older version of the "train TCAS" in operation was in excess of 25km, or 15 miles. That's long. It can be shortened. Sure. But then again, there was such a newer system already in place since 90s that has much less minimum distances than that, which went down.

"Just put airtags on all the trains"... obviously simplistic but I get your point.

> our locations are visible to each other via GPS

... via mobile data or WiFi, you mean. Whether it uses GPS, Galileo, track measurement points, or even "driver calls the control centre to update their position" doesn't have much to do with it. What I assume you mean is redundancy so that two or more systems can communicate train positions independently, where the more relevant problem is how to get the data across in a timely manner. The driver of the stationary train called in their position and it wasn't used: it's a communication problem, not a "where am I" problem. Idk why the focus on GPS and using GPS for communication (which GPS can't do, and newer systems only do at very low speeds for emergency communication) but finding one's position is usually not the problem