Dit werkt:

            $train_id = $_POST['train_id'];
            $user_id = $_POST['user_id'];
                foreach (array_combine($train_id, $user_id) as $train_id => $user_id){
                    $update_axle = $database->summary_add($train_id, $user_id);
                }

En dit niet:

                    $train_id = $_POST['train_id'];
                    $user_id = $_POST['user_id'];
                    $summary_name = $_POST['summary_name'];
                        foreach (array_combine($train_id, $user_id, $summary_name) as $train_id => $user_id => $summary_name){
                            $update_axle = $database->create_summary($train_id, $user_id, $summary_name);
                        }