The online sectional appendix makes a distinction between Mk3 coaching stock, and
HST▸ sets. The former have significantly less restrictions than the latter, which implies, (and this has been discussed in another forum), that the restrictive equipment is on the power cars - and it is thought to be the access ladders, which are outboard of their bogies.
Here's the link:
http://www.networkrail.co.uk/browse%20documents/baseline%20capability/gauge%20capability,%20route%20availability/wessex%20route/table%20d3_gauge%20capability_coaching%20stock_wessex%20route.pdf ...the top of page 4 covering Clapham Jn to Reading is a good example. You'll also see that certain routes are shown as OK for Mk3s but not HSTs, and so must have been cleared since the document was produced, as we know from previous discussions here.
Paul