There are two alternatives for rail travel to LHR to make connections better. One is the Staines connection, where additional tunnelling will allow services from Waterloo/Victoria, Guildford and Reading to run into Staines and then T5. In fact, T5 has the spare capacity to do this.
The other proposal was made by a very well-known construction company, Arup, who suggested that a big train station connecting the M4 corridor services (i.e. FGW HST services) with a WCML branch joining near Watford, allowing fast services to feed people away from Euston-Tube-Paddington-HEX-LHR to WCML-LHR, and stop this monkey business of having to catch a fast service from west of Reading to Slough (requiring a change at Reading) and then the Heathrow Connect, or a fast service into Paddington and then back out on HEX.
Apparently there is some cross-party support for the Arup proposal, and even more support (and the planning's already done and construction could begin soon) for the Staines proposal. If the Staines proposal takes off, then Gatwick Express services could technically stop at Clapham Junction and allow people to switch services there to get to T5 in 35 - 40 minutes, without rolling the dice on whether the M25 will be jammed or not.
The good news about the Arup proposal is that it could connect into CrossRail, making it easier for people to get across town, i.e. from Essex, using the CR to the LHR station, and switch there for a HST to Cornwall or any destination on the WCML, cutting out the Tube and the dodgy services into Liverpool Street.
Oh well.
S.