I read this in the late 1950s or early 60s. Two or three children are sent to stay at a relative’s or ancestor’s house. The plot involves solving a code in which vowels are rearranged or relocated so that “Underground Station” encodes as something like NDRGRND STTN. The message and the plot involve a tunnel, the ends of which are called underground stations.
The Other Side of the Tunnel, by Carol Kendall, 1957.
https://www.goodreads.com/book/show/2192964.The_Other_Side_of_the_Tunnel?from_search=true