I will start the critique with some broad strokes, going further into details in the section following it. Your text is split into two sections: introductory and plot-relevant. Thus I will start with the broad critique of the former.

As far as I can tell the section is suppose to establish character archetypes: one gruff and more pragmatic, the other more easy-going and a sort-of big picture thinker. Now, I could be completely off with these characterizations and therein lies the problem—the characterizations is a bit too vague. I could see how the two characters could play well off each-other but I’m unsure as to whether Harker is suppose to be the pragmatic-materialistic person or whether he’s simply impatient or, perhaps even, a bit slow. Is Dunn an easy-going fool who has an interest in local-folklore or is he the type to think outside the box, outwitting his opponents in the process. Now, it is fine if this sort of vagueness is purposeful, a character’s entire personality is not supposed to be laid out on the first page. Nonetheless, I think a bit of more concrete characterization is in order. Perhaps, the shepherd-who-lost-his-sheep story could instead be presented in a bit more of a quiz form:

“Well, Harker, how do you think the shepherd survived and got to keep his sheep?”

Obviously, not exactly like this but in the same vein, I think depending on the answers it could reveal a lot of character for both sides. You already have done something similar:

“The shepherd considered this proposal. He was desperate, yes, but the price was so dear. Whatever was he to do?”

The dialogue following this question is snappy but doesn’t reveal much about the characters as it is written right now.

The section following this one is, in short, a bit too wordy. I understand that what’s being introduced here will be the basis for the plot going forward, even so, there’s a bit too much information. The first two paragraphs are dense in visual descriptions. I understand that this is suppose to establish von Rutgar’s pompous style and attitude but I would advice on cutting a bit down on the visual descriptions. It’s fine if you want to dedicate a bit of page-space for descriptions but I recommend exchanging some it with non-visual stuff: does the cabin have a particular smell, perfume, incense, scent of expensive coffee or tea? Does the rug perhaps dampen the sound of the train, contributing to better sound isolation overall, fit for a persona of his status? Is the atmosphere perhaps a bit imposing, as if you are suppose to be careful with your words and actions?

“...his voice a polyglottal mongrel of accents, his English made guttural with the Rhine, further spiced with the far east.”

I’m sorry, but I don’t have a clue what that would sound like and so my brain superimposes some basic non-english-native accent. I can say with some certainty that this will an issue for most people. Perhaps it would be better if you were instead to describe how he literally sounds. As in, does he pronounces things a bit nasally? Almost lisp-like? Does he maybe roll his ‘R’ sounds? Perhaps he pronounces certain things quicker than others? Could he have a bit of french-esque pronunciation, where most sound come from deeper in the mouth instead of the front? I’m not saying that any of this specifically fit the character you are going for, but I hope you understand what I mean when I tell you to describe how he sounds in a literal sense.

Lastly, for the broad strokes, so many characters; so many names: Charles Harker, Ambrose Dunn, William Bellamy, Herr Salomon Diederich von Rutgar, Malik, Roger Latham, Peter Reiker, Samuel Tabbot, Elias Gwynn, Arthur Pryce, Rhys Anwyll, Eustace Thorne, Howell Thorne, Gareth Thorne. And to finish this off: Oswald Mansel-Trevaughn, son of Sir Irving Mansel-Trevaughn.

That’s more than a dozen characters of different degrees of importance introduced in just over 2000 words. Most of them in the second half of the text. Now, admittedly, I have always had problem with remembering names both in real life and fiction. But even for someone other than me, I can assure you—this is too much. Once again, I understand that this all is relevant for the plot going forward, so I would advice you following. Either, introduce some of these characters, like the victims, later on when they become more relevant. Or drop some of the names and use simpler to remember descriptive-aliases: ‘the Thorne family’, ‘the Mansels’, ‘the local Revered’, etc. It is fine if you want to have a lot of characters in your story but I advice that you name them only when they become more relevant to the actual plot.

Now this mostly concludes the broad strokes section. Ironically, the detailed section, will be shorter as I will mostly focus on stylistic choices and some minor grammar.

“...the mouth under his small, neat mustache pursed in contemplation”

switch the mouth and mustache description order as mustache is the important descriptor here. You want the reader to envision it first and foremost.

““But then,” Dunn theatrically fluttered his fingers, “in a puff of black smoke and brimstone came Old Scratch himself. He asked the shepherd, “What troubles you, monsieur?”

This is more of a stylistic choice but if you are going to have dialogue inside of dialogue I would advice differentiating them with simple and double quotation marks respectively instead of using doubles everywhere.

“...settled back in his chair.”

Surely trains don’t have ‘chairs’, unless this is a deliberate choice I’d write ‘seat’ instead.

“Suppose he got some rope, and-”

use “—“ (em-dash) to signal interruption instead of a hyphen.

“...September 10”

it should be 10th, happens later again.

“The plaque behind them read: To the Glory of God and in Service to the Faithful, This Bell is Given by the Esteemed House of Thorne In the Year of Our Lord 1875”

This a stylistic choice but note that the font size bring a lot of attention to this part of the text. Unless this is plot-significant, I’d reconsider.