the problem then is that you have 2 shockwave fronts hurdling towards the train, what you would probably want is some emergency seals that could break as close to the train as possible to prevent a huge column of air hitting the train. Basically you would want the pressure wave moving away from the train into the tunnel rather than the train being caught up in it's way, other measures could be bulkheads that close off a portion of the track that the train is in during an emergency repressurization the only problem then is that you are unlikely to be able to stop the train fast enough for this to work. Your worse case scenario is a breach somewhere in front of the train because then you have tons of air hurdling towards the train at potentially supersonic speed + the train itself at 600mph that can't be pleasant.

The valves would limit intake to prevent causing a shockwave, and be spaced close enough to neutralize any shockwaves caused by catastrophic failure.

You could even open the valves intelligently, limit the valves as a function of the distance of the failure point. So the valves closest to the failure open the fastest to put air ahead of the shockwave, and the valves further away open slower to give further away carriages more time to break.