There are some changes being made to the Heathrow Express service. GWR will soon be running these services under a management contract for 10 years and the current fleet will be replaced with some of the new EMU's GWR have recently taken delivery of that will be modified to form a dedicated fleet: -
http://www.railwaygazette.com/news/sing ... rvice.html
http://www.railwaygazette.com/news/sing ... rvice.html