This is one of the legendary classic adventures in the Call of Cthulhu game, and it has the uneasy distinction of being one of both the best and worst written adventures in gaming history. It is incredibly well-written narratively, full of atmosphere, gripping horror, crying pathos. But it is very, very badly written in terms of an actual playable adventure by modern standards. Especially given the almost universally high quality of Chaosium's offerings over the past years, these days even the "updated" edition of HotOE is a notable disaster as a campaign. In many ways, the more modest adventures published for CoC over the last several years are superior, and the only campaign for the game more famous than HotOE, Masks of Nyarlahotep, is roughly 1,000 times better.
The issue, such as it is, lies in both the blessing and the curse of the writing. The authors have created some absolutely AMAZING, truly incredible scenes. Confrontations with villains, episodes of existential terror, desperate battles and flights. They've clearly visualized these, written them in stunning detail, and worked hard to make the players enact them. but that's also the problem - the entire adventure is designed just to force player characters to enact those scenes. No alternatives are seriously entertained. Whatever you do as a Keeper, the way HotOE is written, you must get the characters into those scenes.
At this point, it's kind of a joke: "the railroad adventure is really railroad-y! Ha ha!" but here's the thing: it doesn't HAVE to be. 80% (or more) of the adventure doesn't actually take place on the train, it takes place in cities, towns, and other locations along the Orient Express route. There's no reason those individual chapter/adventures needed to be super-railroady, other than to get the players to those cool scenes the authors wrote. There are entire chapters where the Keeper's main duty is to make the players think their actions make a difference, when in reality you're basically dictating what is happening to them. Heaven forbid your players show some agency, or are clever, or decide not to go along with the program and be captured (I'm trying to write a spoiler-free review here as much as possible, so that's just an example).
There are also numerous optional or side adventures, which mostly are not terrible in their own right, but are rough to fit in to your campaign. Say you're humming along, your players are getting into their groove, really getting into character, and suddenly the main storyline comes to a screeching halt, the story shifts 800 years in the past for a few weeks, and you hand the players new pregenerated characters (or have them make new ones for the couple sessions you'll be playing the side adventure).
To be clear, there is some GREAT stuff in here. It's just hung on a poor framework for an actual, playable campaign. Some of the individual adventures are fantastic; some less so (I'd almost say the huge unevenness in the quality of each chapter is thought by many to be a feature, not a bug).
This campaign is rightfully noted as a classic. Chaosium has mentioned that they're going to do a small redesign in the very near future (just rearranging the current materials a bit), and a large overhaul sometime a few years down the road. Until that time, while this is a fascinating, clever curiosity, I cannot recommend it as a campaign. Pick up Masks instead.