In Batman Begins, the writers were nothing short of brilliant for explaining that the Batcave actually existed before the manor, that an underground railroad was used to free slaves from the south, implying that the house itself was built on that spot for housing purposed... it certainly makes the idea of the Batcave's existence believable.
Did this idea of the railroad and slaves actually originate in the comics, and can someone site the issue?
Like most things in the Nolan movies, it's cribbed from the comics. The exact issue was Shadow of the Bat #45 where Bruce finds the remains of one of his ancestors in the cave.
I think it's mentioned in several stories, though I guess that's the first. Over the years the history of the Wayne family is touched on several times, usually pointing up their benevolence and willingness to go against unjust laws like slavery. I'm vaguely remembering a Judge Soloman Wayne who lived in the 1800s, mentioned in a few stories?
Found it - this says the issue was Legends of the Dark Knight #27 (which makes sense because the first few years of LotDK was the only Batman title I read regularly) - and I remember now it was part of the post Batman 1989 push that remade the visuals of Gotham City in the comics to bring them in line with the movie.