How will the skates know that the one in front of it has broken down and need to be pushed? Especially when it's operating at 120mph?

Yes, I know that. That now means you need a way for the skates to communicate with each other and maintain a safe distance from each other before they collide and obviously slower speeds. That bare concrete tunnel is now longer bare since you will need communication access points, fibre cables for redundancy, etc.

They're going to be running cables for lights. Its not much of a stretch to put in communication cabling as well.

It makes sense to have the skateboards fully connected as part of their 'autonomy.'

Sensors in the tunnel and the skates, that communicate with each other? It's not rocket science, that's how it's worked for years with trains and subways and even car tunnels have continuous monitoring and signage to alert other drivers.

So you are re-inventing a train?

Just wait for a post talking about how they're replacing the tires with revolutionary metal pathways to save wear.