Yep, that's me! Probably ties with the profile view of my face as the Freedmen approached Bohanon as the best shot of me.

Thanks for digging it up!
I only watched the episode this morning, so I've been sorting out my own thoughts. As the climax to the season - or at least the historical goal of the whole show - it seemed almost small scale. But as the climax to a lot of the character work being done, especially for Psalms and his entourage as well as Lee's men (have we even seen that guy before? Not this season, I'm fairly sure) it was satisfying. And Bohanon's reactions this episode from his driving of the last spike (not The Last Spike, mind) were gold, pun intended. Here's a guy who's fought and won and lost so much, and it's OVER. The "NOW what?" question looms large and I'm glad a whole episode will be dedicated to it.
As to my part, I'll relate what I can. Disclaimer: as previously mentioned, I'm a semi-pro actor in Calgary who works mostly improv and live theatre. I have no relationship to anyone in the show apart from my three calendar days working on it. That said, my experiences and opinions are my own.
Pics of me from the episode:
https://www.facebook.com/media/set/?set=a.10153542622555807.1073741869.551330806&type=1&l=5a1c5f3167
After I was cast, I showed up a week or so earlier than the planned shoot days for a costume fitting. It was fun seeing how by the end of five years the crew had gotten their work to a science, shuffling me in and out based on my measurements and finding matching clothes that worked with the visual motif. They had a wall of historical reference pictures to match the costume choices to, and I'm guessing they were fairly accurate to it while making allowances for the fact we'd be shooting in the cold of October 2015 rather than the heat of May 1869. We'd be shooting in the very cold autumn weather and so I was advised to wear long johns and wool socks, but NOT my own t-shirt or undershirt as it might poke over the costume; in the end I wore the full set of long johns they gave me and some extra insulation below the belt.
I was actually supposed to work a day prior to the ones that I did (and got the day off work for it), but due to some sort of snafu I wasn't called even though apparently they could've used me. This must have been for the scenes where Cullen and Mick went out to recuperate their workers from the mine, and for Bohanon's speech at the top of the climactic day as the sun rose against a fairly obvious greenscreen.

Prior even to that, some (but not all) of the background performers were called in and trained on how to build a railway with the tools and materials you saw on the show. I was not one of those, but ended up working with them anyway, being pressed into service as a tie mover.
The first scenes I shot were actually all the nighttime outdoor party scenes following the joining of the railways. This was done on what the crew called "Fraturday", as it started on Friday night and ended the following morning as the sun came up; this was apparently the usual day of the week they used for night shoots and their last day of the work week. The beginning of the evening was thus treated as morning, with everyone bidding each other good morning and the craft services tent dishing up bacon & eggs, which was actually very good.
The Hell on Wheels set is part of the CL Ranch, a working cattle ranch about 40 minutes away from Calgary, where numerous other productions are also made. On the drive in, for example, you pass by the diner used on “Fargo”, and past some pretty conspicuous cabins and old-time buildings that get moved around as needed. We showed up at a cluster of trailers called the "circus", where all the crew do their work, which is just over a rise from the buildings that serve as Truckee and all the other towns (though I didn't shoot in there). We signed in, got into costume, and started waiting until the sun went down (around 7pm). We were divided up into groups of Asian Workers, Freedmen, Irish, and "Fancy", meaning the more well-do-do people in nicer dress. Lots of the performers were regulars and there was a lot of camaraderie amongst them; some of the "Fancy" women were regular prostitutes in other episodes, while some Irish were also farmers or religious types depending on the needs of the week. When the set was ready, we were all bussed about a kilometer away to a holding tent just over a ridge from where the Ogden station was being "built". When it was time, we walked over the ridge and through the maze of cameras and other production stuff to the 200-odd foot of spur that had been built for these scenes.
There were four scenes to shoot over the Friday-Saturday session. The first was some establishing stuff around the bonfire and when Mick was dancing with Eva. There was also some other action with a group of the Fancy people watching the rest of us going nuts, and someone (not sure who) saying they’d never seen such a “den of debauchery” or something to that effect, which did not make the final cut. For this scene I was stumbling around in the far background from camera left to behind the tent and bonfire. Interestingly, while the bonfire was indeed burning wood, it was “supplemented” by a propane flame which had gas lines buried just under the ground.
Our AD wrangler (who managed us Backgrounders while the actual director handled the camera and principal actor coordination) was telling us to party like there was no tomorrow – apt, as we were all now out of work, but now that I’ve seen the episode and know the context of our unemployment, I can see why! Also, at the top of every take they played the same Irish jig music you hear in the episode to help set the mood, but the music cut out just as Action was called.
The second scene was with Cullen and Louise at the cow catchers, which played out as you see it and where you see me in that promo picture (you can see me in the wide shot going from the back of the Jupiter towards the other engine, with another worker holding a bottle). For this scene, those of us who were close to Cullen & Louise were directed to go only so far, then to turn around and stumble back. At the beginning of the shot, you can actually see the spot in the background where people were turning around (before they reached the Ogden station) and coming back. After a couple takes, the guy I was stumbling around with got “promoted” to be part of a Chinese band that was playing right next to Cullen & Louise, and gave me his bottle. Ironically, you don’t see that band at all…
These two scenes each had several takes and then a break was called for most of us – we were sent back over the ridge to the holding area and told to keep quiet as they were shooting closeups for some material (I’m guessing this is when they shot the scene with Psalms consoling Lee), requiring fewer of us. The holding tent had a series of port-o-potties adjacent and they were already getting rank, but there was more than enough trail mix and doughnuts and hot coffee and hotdogs to keep us going. They were handing out hot pockets as well for our gloves and boots, as the cold – especially for those of us not moving – was starting to be apparent. The people you see huddling around the various fires were not acting!
When we were called back, the cameras had been moved to shoot the “turnaround”, i.e. the action from the other direction of the previous scene at the cow-catchers. For this one, I was initially on my same stumbling path but then got pulled to be part of the crowd partying against the locomotive behind Cullen – you can see me back there thursting my arms in the air as part of a silent celebration.
The third scene filmed was that of Collis Huntington burning his trunk full of documents and the Ogden station. While this was also done over several takes, the wider shot of the building on fire was only done once (although when the take was finished, they extinguished everything and could easily have gone again as the fire was made to burn carefully on specially treated wood. I was part of the group at the bonfire just beyond the building, but you can’t really see me there.
Lunch time followed, and after we were all bussed back to the circus, I opted for a half-hour nap and a couple granola bars over what I assumed was some delicious chicken curry. We quickly bussed back keep going on the final part of this marathon nighttime shoot.
The final scene shot was the exchange between Bohanon and Durant (“I won”, et. al.). For this shot, we were directed to be extra-drunk and slower as we’d have been partying all night, but as we’d been shooting all night, it certainly wasn’t hard to be tired and slower! I was with several people from all the groups partying against the Jupiter engine, but we aren’t seen (though the Chinese band are in full party mode behind Bohanon and Durant)…The scene actually went on a fair amount of time after Bohanon walks off, and he was supposed to disappear into the crowd as several of us went to pat him on the back in congratulations, but this was all cut. In the final couple takes of this though he actually stumbled and fell to the ground, drunk, before the action was cut. One of these takes was fouled by a Background guy partying right through the shot as the actors were finishing up. After the AD talked with them, this was not repeated!
There were a few more takes with the closeups, and while most of us would not be seen we were instructed to hang around in case we’d be needed for alternate takes and angles. With that though, we were done as the sun was peeking, just before 7am. After getting changed and finishing off the paperwork, I drove off to sleep off the rest of the day, weary but still fulfilled as I could be on this experience.
I’ll soon post my notes and thoughts from the third day of shooting, which involved the various scenes of people racing to Ogden on the final day. In the meantime, I've got day-job stuff to do.
Mark