The existing 1920s track circuit technology is simple and reliable. The train's wheels short the rails together, cutting off power to a relay at the end of the block, which sets signals behind the train to red. The classic system also detects broken rails and cars that are not part of trains. If someone leaves a freight car someplace, which happens, it's detected.

The classic system doesn't need GPS. The safety components ("vital" in signaling parlance) are very simple. The safety-related components are in trackside boxes along the right of way. There's lots of additional gear associated with dispatching and switching, and that's often centralized and computerized. But the basic train-presence detection and signal-setting is local.

There are later systems that still use track circuits, but provide more information to the driver and locomotive. These involve devices which sit between the rails and send signals to the train. Most of these are passive RFID-tag type devices. Some are wired into the signaling system.

The classic gear is old, but has a good track record.

Never buy trackside equipment from a place with a better climate than yours. - old railroad adage. The better railroad equipment suppliers used to be in upstate New York or Pittsburgh, where ice and snow are a normal part of life. Railroad equipment has to survive powerful snowplows, stuff dragging from trains, flamethrowers used to melt ice from switches, lightning strikes on the rails, machines which replace ties and ballast, and routine minor vandalism. There's a reason that gear is packaged in cast steel boxes.