So it worked out pretty well! The party came in via the rail tunnels (they travelled from the next station), and used some good tactics to distract and destroy most of the terrorists, with most of them getting on the train just as Zubov + Prophet got it moving. They were pretty on the ball about stopping the train from moving (I don't recall if Zubov was meant to do anything to stop the breaks from working, but eh, it works). They dealt with the dwarves pretty swiftly and moved on to the bomb itself.
So Celawyn was mostly on his own on this - he got a few luck spells, but no assists; the only person who could ably assist him decided to flee the scene immediately (Boris is pretty self-centered and self-serving).
I gave them the general description of each component, that cost them 4 minutes on the clock and two Arcana DC27 checks (the one failed, the second one succeeded). Celawyn initially suggested just cutting Fire Humor, I let him know for 1 minute on the clock that a failure there would result in a full bomb ignition.
He then had a good think about the other components, rolling out the more flavourful ones and weird ones like the Blood Infused with Ambergris. In the end, he cast Cat's Grace and used thieve's tools, and without even needing the luck cut Aether, Phlogiston, and then finally Fire Humor. All his rolls were in the 25+ range. Celawyn was still very tense and everyone was breathing a sigh of relief afterwards. They then went upstairs and punched the Colonel (cause the Colonel is an arse).
It was interesting hearing what they expected - Boris's player theorized that the first DC would be tough, but it'd get easier as it went along, the very inverse of how the original encounter was written. I think I liked this version a lot more though; it meant that the players really thought about each component and how the bomb was made, so the bomb plans found have a bit more meaning.
