(Warning: This post contains some extremely fannish arguments and pedantry. I'm also going to avoid spoilers, only because I don't think my argument is supported by my revealing plot points. Proceed at your own peril)
To be honest, I'm not quite sure how I feel about Mummy On the Orient Express.
On the one hand, it's a good looking episode, from the set design to the special effects. Even the countdown clock in the lower left corner looks good. Some clever touches - a cigarette case full of jelly babies, the best use of the "Are You My Mummy?" line from 2005 - add to the overall feel. If this were a serious romp, it would be even more enjoyable....
But after Kill the Moon....well, it left a bit of a taste in my mouth. It seemed a little too eager to say "let's-not-explore-the-repercussions-too-closely". I also think - and this is where the fannish pedantry comes in - that I can better explain why I feel the way I do.
In their review of Kill the Moon, the Verity podcast (or more accurately, one host who I happen to kind-of sort-of know in real life), put forth the argument that Capaldi's Doctor is the production team's effort reflects an effort to take on the New Adventures Doctor. For those not in the know - after the original series ended in 1989, Virgin Books released a novel series focused on the Seventh Doctor with "adventures too deep and too broad for the small screen." In this series, the Seventh Doctor was viewed less as benevolent alien fighting monsters and much more of a distant, almost godlike figure. One who continually drifted towards an attitude that the ends justify the means. And the New Adventures Doctor had a frayed relationship with his companions, who would consistenly remind him - a non-human - that people have value. (Yes, this is a very simplified explanation, but you might want to consider listening
to the Doctor Who Book Club podcast as they provide insight into some of the New Adventures).
My experience with the New Adventures is limited - I did read some of the books (and have several as ebooks) and the only one I really enjoyed was Paul Cornell's Human Nature. (Trust me, it's just as good as the Human Nature/Family of Blood two-parter in Series Three). I don't need the Doctor to always be on the side of right - or even on the "right" side - but I think the Doctor works best when making moral choices, including those which the audience might not agree with. Thankfully, Mummy on the Orient Express clarifies it somewhat - although there is a sense that this Doctor is being softened (after all, I still maintain the arc of this series is whether the Doctor determines whether he himself is a "good man"), there were too many bits that felt....well, shoehorned an inappropriate. The end result is a Doctor who says, in essence, "Yes, I'm a jerk, but all that stuff that happened before - doesn't matter, because I'm going to be a jerk until I decide otherwise."
The return of the "soldier theme". The "Is that what you would like to believe?" moment. A lie that is told that really doesn't need to be told. The nod-and-wink "Bechdel test" moment.
I don't mind romps that turn dark - I actually kind of like Dinosaurs On a Spaceship - but Mummy on the Orient Express seems a bit of a cheat. It seems like it's trying to both darken the Doctor and excuse his behavior. I'm willing to stay on - after all, the return of a non-split season means that the overall arc needs to play out fully - but this is going to be one of those "maybe in the future, I'll like this episode a lot more than I do".
Because right now, I don't.