I was wrong. Clara is still around in "Mummy on the Orient Express", despite the revelations of "Kill The Moon". It feels like an episode was skipped somewhere that would have dealt with The Doctor as he has to travel alone for a while - instead, we get an awkward apology and the sense that one of the most important events in their relationship (The Doctor contacting the Clara to apologise or to try and make amends) happened off screen.
"Mummy on the Orient Express" takes place on the only train called The Orient Express to travel through space. It's a 'last date' type affair for Clara and The Doctor after their argument in "Kill The Moon", Clara's way of ending on a good point (or so she keeps telling herself). But, as is prone to happen whenever The Doctor takes his TARDIS somewhere, it's not long before they are in the middle of murder mystery type affair, with victims claiming to see the decaying remains of a mummy walking towards them exactly 66 seconds before they drop dead.
The cold open does a fantastic job of introducing the premise, showing us the demise of an elderly woman, screaming frantically about the monstrous horror that only she (and the viewers) can see, while everyone else looks around, presuming that the old dear has finally gone mad while the timer in the bottom corner of the screen counts down to her inevitable death. It's a great opening to the episode, as instantly captivating as The Doctor's speech at the start of "Listen", drawing you in to an otherwise fairly ridiculous premise.
The Doctor takes charge here, and it's the first episode of the entire season where I've really felt that Clara was a secondary character, a nice change in a season that has been utterly dominated by her so far. We get to see more of Capaldi's Doctor, and although we already know his basic personality by this point, we do gain further insight into his way of working and his general ability to take charge of a situation simply because he finally gets an episode in the spotlight, and it does show us the The Doctor isn't really as heartless as he as appeared in other episodes - rather, he just realises that no one has time to mourn when the clock is ticking - rather literally in this case.
It's just another 'monster of the week' type episode at the surface, but I'd be very surprised if it didn't turn out that the entire affair has been orchestrated by the mysterious Missy. Again though, it is nice to see yet another episode of Doctor Who that allows an episode to be a self contained story, and doesn't require a vast knowledge of previous seasons to be properly enjoyed, my main complaint with the Matt Smith seasons. The plot is started and finished by the time the episode ends, and although extra information will no doubt be given later in the season, you could happily watch just the episode and still be entertained.
If I have just one real complaint, it's that Frank Skinner was completely wasted on this episode, similar to how Ben Miller was wasted on the truly terrible "Robot of Sherwood". His character has very little to say and even less to do, and I strongly suspect that he wouldn't have been in the episode at all without a celebrity playing him. Unless his character shows up again later this season (and I wouldn't rule it out - he seemed a fairly suspicious character at first) then this is another example of wasted potential - and it has me worried for Nick Frost's upcoming appearance in the Christmas special.
Overall, "Mummy on the Orient Express" moves at a real pace (something this season seems to excel at), and as such never gets bogged down in the technicalities of the episode, which works in it's favour. It's not a great episode - it will end up being one that you catch on TV in a few years, rather than one you specifically seek out to rewatch - but it is another episode that does enough right to keep the season moving forwards.