I presume you are referring to the entry for 6th October 1935 which says "SJ SB closed. Crossover removed. Mail coaches now reverse round triangle clockwise instead of anti-clockwise."
However, there is also an entry for 1926 which reads: "SJ SB replaced by remote controlled points operated from MS SB."
So we are given two possible closure dates for South Junction box, 1926 and 1935 (incidentally, for those who haven't looked at the link, SJ = South Junction, SB = signal box, MS = Mangotsfield station)
The entry also says "crossover removed." However, the 1:2500 OS maps that we have been looking at do not show any changes in the track layout at South Junction. I would never say that OS maps are always 100% correct, but it does introduce an element of doubt into the apparent reason for the change.
All that said, it matters very little after all these years which way the mail was turned on the triangle at at Mangotsfield
But the main reason for posting this morning is to respond to these comments:
Matt Thomas wrote: Why was St. Philips Marsh, so much more convenient for the centre, not preserved; (Old Market, I think, if you go back far enough, used to be the commercial centre of Bristol)
You have to remember that this was a different age of the world. St Phillips station was not put there for passenger convenience, it was put there for railway operating convenience as TM could not cope with the traffic.
It is an unfortunate fact of life that many areas of the railway's management, at least up until the 1960s/ 1970s, did not understand how commercial businesses were run, what the ever-increasiong road competition could and was doing to railway revenues, and/or what their customers actually wanted.
In 1953 all the railway management could or would see was that, as Temple Meads could now cope with the St. Phillips traffic, then the latter could be closed to save some money and all trains could be diverted to TM. They could or would not see, as you have already mentioned, that Old Market/ Castle Street area was the major commercial centre of Bristol before the war and, after the Luftwaffe had done their bit, the new Broadmead shopping area was even further away from Temple Meads than the old one (OK, not a lot further away but read on

)
I am told that I used St Phillips station many times in the last 15 months before closure but, being in a pram in the van along with the numerous other babies being taken into town in this way, I don't actually remember much about it
What I do remember from conversations with my father, however, is that he told me that the closure of St Phillips resulted in the vast majority of the remaining suburban traffic in north east Bristol draining away to the number 4 bus, because that did go to Old Market Street, and people were not prepared to take the extra 10 minutes walk and go by train into TM.
What is especially significant about this is that my father worked on the railway and was entitled to privilege tickets, and if even he stopped using the train to get to Bristol after St Phillip's closure, it speaks volumes about what the ordinary fare paying passenger was thinking and doing.
Matt Thomas wrote: And how much money would the Bristol-Bath/Bristol-Yate lines be making today, had they not been abandoned? (let us close our eyes for a moment and imagine Mangotsfield Parkway, with the Carsons cricket ground as a car park and half-hourly 125s).
[Unfortunately, a combination of greed (too much money in building roads) and lack of foresight prevented this dream from becoming reality.
Whilst, as I said above, there were many things wrong with railway management's thinking in years gone by, it is also possible to apply 20/20 hindsight to come to conclusions that could not reasonably have been foreseen by a more commercially-aware management. The following comments could be said for any area of the country but, as we are talking about the Bristol to Mangotsfied, I'll use that as a specific example.
As they saw it, railway receipts for both passenger and freight traffic had been falling since the 1920s. Trams and later buses had taken most of the suburban traffic away. Private transport usage had also grown, especially after the end of the war, when many men were demobbed having been taught to drive while in the services. This was seen as a permanent change - nobody was using the railways any more, they were yesterday's form of transport.
So what were railway management thinking? Temple Meads was seen as not being able to compete with the local bus network because it was incoveniently sited for the major shopping area. St Phillips was better sited but that had to go because we didn't need two stations in central Bristol if one coulld cope with all the traffic. The major population centres in Mangotsfield were nowhere near the station, but very near the number 4, 97 and 319 bus routes. The only really local source of income generation for Mangotsfield station when it was open was Carson's factory, and the income from passenger traffic from that wouldn't have paid the station staff wages bill.
Nowadays, of course (and really only in the last 10 years) we have seen major residential developments very close to Mangotsfield station. But, even if it were still open today, how many of the local residents would actually use the railway facility on their doorstep? Personally I rather doubt that many would.