This is illogical, Captain.
See attached sketch: there were three routes (red) available after passing SN109: first, to the down (points 1, the route the train was intended to take); second, to the up (points 2, the route it did take); and third, to the down again but some distance further on (the route it would have taken had the flank protection been intelligently implemented). The only place in such a layout that a sand trap could have been installed, both from the viewpoint of physical possibility and that of making sense, would have been as a straight-on continuation (via trap points, points 3) of the third route before it joined the down (mucky yellow colour "Sand?"). For it to work, points 2 would have had to default to the normal position - which is the same as required for flank protection without the sand trap. So for the omission of the sand trap to result in the loss of flank protection there would have had to be a deliberate decision to change the default state of points 2 at the same time.
I remember this being extensively discussed in RAIL at the time - which memory is the source of my sketch - but I don't remember anything being concluded as to why it was the way it was; I was just left with the impression that nobody had thought of it.
I don't remember anything in the report about why it was the way it was, either. In fact I thought the report was severely deficient in the way it skated over the matter. As I remember it there was no more than a page or so, maybe less, in the whole vast tome discussing it, and a one-sentence mention in the recommendations for action which was expressed in very weedy terms.
Given that sorting it would have been a single-bit change, and would have prevented the accident (though for sure at risk of a less serious one), the lack of attention paid to it was badly negligent.
The report also gave the strong impression that a large though unstated part of its purpose was to minimise political embarrassment by deflecting criticism of the deficiencies introduced by privatisation. Bluntly, the Thames Trains driver was not a railwayman. He was a chap who had come in off the street, been crammed through inadequate training at high speed - inadequate particularly in its coverage of the routes out of Paddington, and also inadequate in its assumption that a period of high speed cramming is a valid substitute for the sense of responsibility and overall understanding of how railways work that comes from years of actual experience - and then effectively chucked in at the deep end and expected to work trains over a complex route which he didn't know properly and which was confusing even for long-served drivers. He then proceeded to make several mistakes in succession, each compounding the effects of the last:
- missed the signal
- cancelled and ignored the AWS
- continued over a wrong route
- accelerated when he was already in a situation that he should have known to be out of order
(It appears to my mind that he was likely thinking in terms of driving cars, where "put the hammer down and hope", while not to be recommended, nevertheless does often work as a means of retrieving an embarrassing situation after you've cocked up, and failing to appreciate that on the railway it is by contrast a guarantee of disaster. Trying to put myself in the same situation I can certainly appreciate that there is a temptation to do it.)
All this was in the report, but very much de-emphasised. There was far more coverage of the sighting difficulties with SN109 than of Thames Trains's recruitment and training procedures. Of course the sighting matter was important, but the reason it had such a disastrous effect was that several incorrect actions were taken after missing the signal, and the responsibility for these actions is down to the driver himself and to the inadequate training and lack of experience he had been given by Thames Trains. But the way the report presents it is to focus an inordinate amount of attention on SN109 and to pretty much shrug off the matter of inadequate training and inexperience as "that's just how it is these days, we can use TPWS to sweep it under the carpet".
Of course, if the report had put an appropriate amount of emphasis on training and experience, pointing out that the changes of privatisation had resulted in Thames Trains - and others - deploying inadequately-trained drivers on a large scale and being emphatic about the need for improved training of drivers and their recruitment only from among those who had already worked on the railway for long enough to get into the habit of "thinking like railwaymen", there would have been a massive outcry from politicians, TOCs and passengers alike. But the purpose of such reports is to identify causes and recommend measures to prevent their recurrence, and to de-emphasise those aspects of the causes which might embarrass someone is to negate their own purpose.
(I'm not denying that experienced drivers sometimes also do stupid things - the recent Tangmere SPAD is a case in point - but there are a whole slew of incidents which we never get to hear about because the driver has reacted correctly after realising the initial mistake and so avoided any deleterious consequences. I think it should go without saying that an experienced, long-served driver is considerably more likely to "fail safe" than an inadequately-trained raw recruit.)