You're Right Guard ko to pata chal hi gya hoga but Air break ka zamana h aur Break Pressure ka supply loco se hota h agar train k break pipe me pressure nhi hai to Break Supply poore train ki cut ho chuki hai agar Guard bhi Vacuum break lagata h to bhi train nhi rukegi he is also helpless in that situation because at time train k break pipes me air pressure tha hi nhi