Could be that a more productive approach might be to try and work out where that is first and then relate it to the railway, rather than the other way round. Not sure how much I fancy reading the whole book though.
He calls it an "ugly little town", but I've seen that description from plenty of authors and unless they're talking about some grim northern industrial dump it never seems to be true, unless it's because of buildings added a long time after the book was written/set.
You can see right up the main street from the train while it is performing the manouevre, and the view includes a house with a "memorable" facade.
The journey to Salisbury then seems to be fairly short and without any further notable features, although it does stop at at least one intermediate station and appears to cross the border into Wiltshire.
You get a good view of the spire from the train approaching Salisbury. I don't know if that helps at all; it might be true of all the lines into Salisbury for all I know.
A faint possibility might be West Moors:
http://maps.nls.uk/view/106009731 You'd hardly call it a town or even much of a village, but it does look like you could see right up the main street (such as it is) from the railway.
There were through trains between Bournemouth and Salisbury by the Ringwood route despite them having to reverse at both Ringwood and West Moors. Ringwood seems to have been a pretty well-equipped station, but the layout at West Moors was pretty minimal; if it was necessary to shunt the Salisbury train over to the up line before sending it down the branch instead of reversing it directly in the down platform then that might be the manouevre described.
However, I'm not at all convinced he didn't simply make the whole thing up, and just throw in an idiosyncratic manoeuvre he'd seen performed somewhere or other for a bit of extra colour.