There are three train-riding experiences at Didcot. One of them is a short stretch of Great Western Railway broad gauge (7 foot, as opposed to the 4' 8 1/2" standard gauge) line. Here we're being pulled by the replica of early GWR engine Fire Fly, on its last day of operation before a planned boiler rebuild. We're riding here on a replica of a third-class passenger carriage, such as one would have ridden in a goods (freight) train (because only first- and second-class carriages were in passenger trains). Yes, it's open to the elements; that's how they rolled in those early days. Note that the footplate crew running the locomotive didn't have it any better themselves.
Moving forward a bit in time, we shift to the "Branch Line," which replicates what a typical GWR branch in the 1930s was like. It starts in the Transfer Shed (where you transfer between the broad and standard gauge lines) and runs past two signal boxes to Didcot Halt.
We had a fine time at the Didcot Centre, despite the rain that was heavy at times. In fact, it's only time I needed my jacket this whole trip. We did get a little bit damp. Our main regret was that we couldn't stay longer than we did, on account of needing to get back to London to get out to the airport.
Edit, September 15: Here's the still photos from our day at Didcot, including the trains from Paddington and back.
We were fortunate on our return trip, as we'd forgotten to check return times, but there was another fast service only four minutes after we turned up. We did go ahead and spring for the Weekend First upgrades again. (Today is the August Bank Holiday, so Weekend First still applies.) This was particularly good when we made our only intermediate stop, that being at Reading, where a horde of people poured onto the train from the Reading Festival. Few if any of them went to First Class, and we had a pleasant ride to London.
Then things got a bit stressful. In the process of working out a step-free route to Paddington, we found a route, but it took a long time, and it's by no means the direct Underground route. The route is Southwark (Jubilee Line) to Westminster (District) to Hammersmith, then leave the District Line station and cross the street and rejoin at the Hammersmith & City Line station back to Paddington.
Once at Paddington, we bought some food intended for our dinner when we got to our hotel, the Holiday Inn Heathrow Ariel, then boarded Heathrow Express for the short ride to the airport. Had we really understood the situation on the ground, we wouldn't have bought the round-trip HEx tickets, but would have taken the Piccadilly Line to the airport. The roundabout routing to handle our heavy luggage meant that any time we saved on HEx was lost on wandering the Tube.
Trouble wasn't over. The Heathrow T123 bus station people told us that to get to our hotel, we needed to go to T5 and catch a bus from there. We did that, and finally got on Hotel Hoppa bus there, but it turns out that there is a Hoppa bus that serves T123 where we need to be tomorrow. That wasted at least another hour or more.
I took lots of photos at Didcot. I have not processed any of them and won't do so tonight because I must get some dinner and then to sleep, because by my reckoning, we need to be on a 6:30 bus from this hotel to be at the terminal three hours before our flight. It's just as well that our room rate doesn't include breakfast; we'd be tempted to stay long enough to eat it. As it is, if we get through the check-in/terrorization process sufficiently early, we should be able to get breakfast airside.
But now I need sleep. I expected to be in bed long before now before doing the Heathrow Shuffle.