A railroad apartment is like when you play with your toy train set and the train goes through a bunch of different rooms. In a real railroad apartment, instead of a train, people can walk through different rooms to get to the other end of the house.
First, you go through the living room, which is like the first stop. Then, you walk through the bedroom, which is like a second stop. And finally, you reach the kitchen, which is the last stop at the end of the apartment.
The rooms in a railroad apartment are all lined up one after the other, like train cars, and they usually don't have any hallways or doors between them. So you can see all the way from the living room to the kitchen without any walls blocking your view.
Railroad apartments are usually long and narrow, kind of like a train car, and they are often found in old buildings in big cities. People like them because they are cozy and unique, but they can also be a little bit tricky to decorate and live in because of the long narrow shape.