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.
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.