> Previously, each train could automatically identify its proximity to another one. “It is designed to stop so trains don’t collide,” said Pérez Varela. Now, human operators, who had initially been hired as a fail-safe to manually halt a train if the automatic pilot failed, need to intervene more often. Trains usually run at 70 kilometers per hour, but when the autopilot signals that it is not working, operators must take immediate action to manually reduce the speed to 35 kilometers per hour to avoid derailment. > > Since the trains are increasingly manually driven, real-time communication among operators is essential. “We are all provided with a radio but it doesn’t work properly,” Jaime, an operator who has worked at the Metro for 17 years, told Rest of World on condition of anonymity. “That’s why we started using our cellphones.”
That reads to me like they need communication to ensure they don't run into the rear of the train ahead.
Those people might be right. But I'd like to point to the old solution, the radios in the trains, which are in the middle of a decade long outage right now. It's more reliable than that.
And this stuff isn't safety critical - in-cab radios are mostly used for logistics. Everything from scheduling shifts to getting trains to wait a few extra minutes to even out gaps in the service. Without it, the service wouldn't run so smoothly. But it could still run.