Coupling / decoupling at high frequencies would be a challenge for sure - but seems achievable in a world of fully autonomous vehicles that can communicate.
I believe most AV convoys propose virtual coupling to get around this, but the failure scenario where something in the convoy comes to a sudden stop and the other things in the convoy don't, is pretty gnarly. Modern trains have a bunch of mandated safety measures in their mechanical coupling to prevent nasty things like jackknifing, or even worse, telescoping https://en.wikipedia.org/wiki/Telescoping_(rail_cars)
Generally speaking mechanical coupling takes a while because you really, really do not want an errant car to detach during its journey. In the worst possible case scenario, a rear carriage has decoupled from a train and then the following one has crashed into said carriage. https://localtvkstu.wordpress.com/2013/11/05/crews-respond-t...