In the short story The Girl in the Train, Rowland's Castle is a location which is reached by train from Waterloo Station. George Rowland decides to travel there, after being thrown out by his uncle. Since it is named after name (so he thinks), he assumes that he will have a welcome there.
The location of Rowland's Castle is not stated but George departed on a train of the Southern Railway from Waterloo which ended at Portsmouth. Other stops along the way (presumably before Rowland's Castle) are Woking, Weybridge and Wimbledon.