As other reviewers have commented, this book has two strikes against it. First, Sayers transcribes most of the dialogue preserving the native Scottish accents of her characters. Occasionally she'll allow a character to have so thick a brogue that she'll simultaneously translate for the reader. However, it frequently takes several times through a conversation to make sure you're reading it properly. A glossary at the end of the book would have helped immensely (everybody say Imph'm). The other strike against the book is that five red herrings is a couple kippers too many. Combined with the dialectic nature of the book, there are simply too many people (suspects, police, railroad employees, servants, etc.) to keep track of at the same time.
Fortunately, Sayers doesn't get the fatal third strike. She weaves a complex web and sets master sleuth Lord Peter Wimsey in the middle of it. The obvious wit in her other novels is obscured somewhat by the accents, but enough shines through to keep the overall tone light. Bunter disappears about halfway through, but while he's on the scene he's as wonderful as ever. Tracking Farren and Wimsey's re-creation of the murderer's alibi were, for me, the high points of the story.
I'm sure Dorothy Sayers knew the risks she was taking in crafting such a detailed, complex mystery. That it doesn't entirely work for an American reader in the 21st Century probably isn't ruining her afterlife much. I've found myself hopelessly outclassed on several occasions when reading the Wimsey series, and under those circumstances I find it most helpful to get in Wimsey's Daimler with him and go along for the ride. The trip is always breathtaking (as most of Wimsey's passengers can attest), and while Lord Peter may know where he's going sooner than I do, he doesn't get there too far ahead of me. Don't let my criticisms of this book dissuade you from giving it a read; it's tough in parts, but well worth the effort.