All trains already have the ability to tow and push other trains. That's what the couplers on the front and rear are for. The train behind the disabled train simply connects to it and pushes it to the yard.

Buses don't have that ability. When a Silver Line bus breaks down in the Waterfront tunnel, the buses in front of it have to clear the tunnel, and a mini tow truck has to back into the tunnel. It takes way longer to remove a disabled bus from the tunnel than a train.

The problem isn't the technology. Trains are already proven to be more efficient than buses in other first world countries. It's that the MBTA's operations are shit and they manage to make both trains and buses wildly inefficient.