In an emergency brake, you [i]do[/i] slam on the brakes 100%. And communications doesn't matter. For two reasons. First, communication can fail. There's no such thing as a 100% failsafe wireless communication system, so you should not be driving in a configuration that requires 100% perfect communication to be safe. Second, the system you describe requires that the lead car(s) not brake as quickly as possible. So you could easily end up with a situation in which the lead car could have stopped in time to avoid a fatality, but couldn't because of the choice to operate in a train. That's a decidedly idiotic result.
Furthermore, braking isn't the only potential scenario. If the lead car swerves, and the car behind is two feet behind, it may not have enough time to swerve. And, similarly to braking, wireless communication cannot be relied on as a solution to that scenario.
The situation you describe is simply unsafe. No amount of futurist handwaving will change that.