They can’t book 75 people for a train which can only seat 50

The official statistics lists how frequently that occurs.

+1. “oh you’ll surely be good if you are early, the train can’t possibly already be overcrowded when it arrives”

I suspect the sales website can’t actually reserve seats itself, but just passes along the request to some other system, which enters “LOL, NO!” in that field for a train that was long-since fully booked.

My guess is that it printed this “null” reservation slip to let you know that the reservation had failed, because otherwise people would think that the printer wasn’t working? It prints the ticket(s), then the reservation(s), then the receipt listing how many things were printed.

It’s not possible to prevent overbooking - anyone with an open ticket could get on this train, and the train company wouldn’t even know they were coming.

They only issue as many tickets as seats

That’s… optimistic, but we can be hopeful.