Check the (historic) timetable!
I blithely set part of my novel on the Isle of Mull in April 1952, with relatively little research or planning. Max flies to Mull, but after a plane crash, she has to take a train back to London. Somehow I thought the steamer that went from Mull to Oban would match up with the train timetables, but upon research in Bradshaw's for April 1952, I learned it absolutely did not. Moreover, I also realised you couldn't take a through train from Oban to London on the weekend, so the timings of the novel required shifting. But the research was fun to do!