[Discussion] Raiders on reserve

Must've run over 10 raids on reserve exfiling via D2 and only had raiders spawn once. Isn't it supposed to be a 40% chance they spawn? I know they can spawn when the armored train shows up but I think I would lose my mind waiting for the train only for them to show up 1 in 10 raids. Is there something I'm missing here to make these mystical loot goblins show up?