Great Western Coffee Shop

On the 18:30 London Paddington to Western Super-mare and we were told that the train needs to reverse!  Apparently there are two London bound trains in the platforms in front of us and nowhere for us to go.  Driver was told to go to the rear power car ready to reverse to the nearest crossover so we can go to a different platform.

Guard on the train said that people up high were scratching their heads to work out how this happened.

We're now proceeding very slowly into Reading without reversing - the driver had to walk down the tracks to the rear power car and then again back to the front so they've obviously made one of the other trains reverse out of the platform.

Lucky that it happened at the platform and not halfway between stations!

So for a second night running, anyone wanting the 20:18 to WSM presumably were inconvenienced.  This could have been a potentially serious incident, so I hope we get to the bottom of it.  Better to be scratching heads, though, than having to issue condolences.  Someone perhaps deserves a commendation for realising the error in time.

On JourneyCheck, the 18:30 is showing as being delayed "due to a broken down train".

Well, just from what's in RealTimeTrains, it seems:

P10 is busy, as usual,with up trains
P7 is blocked by a leisurely splitting a turbo in half operation from 18:47 to 19:45 (due 19:21)
P9 is blocked by the failed 18:45 to Swansea (until sometime before 19:48 - not shown)
P8 has a lot to do, but the XC to Bournemouth gets away at 18:51 (only 5 down)
next the 18:53 Hereford gets through at 18:56 (6 down)
next is the 18:53 from Penzance, waiting for its slot, going the "wrong" way - it got in at 19:40!
but the 18:54 from Paddington to Weston is waiting at the other end - and got in at 19:23

My guess is that both trains got so far that the exit of the other was blocked, and the Penzance train was made to reverse,
 or maybe the section was entered but in the end it was decided the Weston train could squeeze through without any reversing - is that allowed?

In fact, another down train (19:04 to Exeter) is shown as in P8 from 19:29 to 19:41 - so the Penzance train must have waiting somewhere all that time (but not blocking entry to P1). Note the times as shown for P8 do overlap.

I was waiting for a train on platform 8/9 today, and your explanation sounds likely. When I arrived, after 1856, a train was in platform 8, and the Weston train was just outside the station. Once it left (country-bound), the Weston train arrived quite soon after. Thus, I expect the mystery train I saw was the 1853 from Penzance.

There is some further desk-based evidence, in the track layout. There are two routes for the Penzance train from the Up Westbury to P8, one (presumably the usual one) leads nowhere else, the other is to approach P9 and use the "high agility" link to P8.

The conflict point for the usual route at the country end is at the points right at the entry to P8, while at the London end it is back at Kennet Junction. Thus the Penzance train does not block the Weston train unless it is actually in the platform, and the Up Westbury it would wait on is marked as bidirectional (but not fully signalled). The Down Main approaching Kennet Junction is not bidirectional, so seems a less likely candidate for a reversing manoeuvre.

The following down train was for Exeter, due 19:01, running 3 minutes behind the Weston train, which also makes reversing here an odd choice. The next train on the Up Westbury was from Basingstoke and reached P1 only 6 down at 19:06, on a path using the Down Westbury from well back. However, after that there were an XC from Bournemouth and a Newbury train, both of which got to P10 before the Penznce train was finally cleared (at 19:08 and 19:26).

Now the Penzance train was originally meant to use P9, before that was blocked, so its path would be as for P10 from Westbury Junction, not as usual for P8. So did it actually approach P8 that way? That would conflict with the Weston train from further out, as well as blocking the next two Up Westbury trains behind it. So perhaps it was taken into P8 to be reversed out onto the bidirectional line direct to Westbury Line Junction, which is long enough to park it while the stacked trains both ways get past.

Note also that the up path on leaving P9 and P8 join well before the conflict point at Kennet Junction. So the platform switch is no reason for the Weston train to be held too far forward.

That appears to be the limit of this desk-based approach: RealTimeTrains shows the Penzance train as waiting somewhere past Southcote Junction before entering P8 for 50 minutes - nothing about it going in and out and in again. What's needed now is some HUMINT to fill in the lacunae in the ELINT. If this report is of the Penzance train:
where did it go at the platform exit (left or right) and how far?