Or, as FDF suggests, via LHR - though it's not immediately obvious where they would continue to or what would be the point in that.
Maybe Reading-London nonstop alternating with Reading-London via LHR-5 and LHR-1/2/3, the latter being similar to the way that trains from south of Amsterdam do a stop at Schiphol on the way to Amsterdam.
Alternatively the trains could terminate on the airport (LHR-5, LHR-1/2/3 then LHR-4), but they may as well miss LHR-4 and continue up to London and pick up some Heathrow-London pax.
Much would depend on the numbers of pax using this, but expect those who use the bus from Reading and those who double-back from London may find a through train convenient, and who knows, the conveniece may even generate some new traffic.
Having spent millions on building the link, it would be crazy
NOT to use it in this way (even so, don't hold your breath!).
That, to me, would be the point.
As a UK taxpayer does it not perplex others that they're spending millions of taxpayer £££'s on a link that, post-Davies review in 2015, could be completely redundant if LHR is not chosen to construct R3?!
A private company wouldn't countenance that kind of spend with such uncertainty... or maybe Network Rail know something we don't!
Good, all that's needed now is for the government to grow a pair.