Secret Agent Grandma (Give Yourself Goosebumps #16 by R L Stine
Your parents are going away so your super cool Grandma is coming to stay with you, but when you go to meet Granny at the train station, you start seeing double Grannies. One at the station platform, another one writing in lipstick on the window of the train. Which one is your real grandma? If you think she's on the platform, you find face to face with a heinous monster! If you decide to jump up on the train, you are surrounded by a group of angry aliens out to take over the world.