call into question = to challenge the accuracy, probity, or propriety of something Example: "We must call into question your judgment of this serious matter"

To "call into question" means that it forces a reevaluation of something or calls something into doubt. In other words, it makes people question something that they had previously taken for fact. In this instance, it's making people doubt that bullet trains in China are as safe as they had been considered before the recent train accidents.