Basically:
- It goes from Paddington - may be signed as "Hayes & Harlington".
- It has a separate price structure and ticket desk (i.e. cheaper at £6.90).
- It stops at Ealing Broadway, for connections to the Central Line.
- It goes every 30 minutes, instead of every 15.
- It takes 28 minutes, instead of 15.
- Travelcards are valid up to Hayes & Harlington.
- It only goes to Heathrow Central, not T4.
- It goes from the same platform as the Heathrow Express at Heathrow Central, signed 'Stopping Service to London Paddington'.
Details on its website at
http://www.heathrowconnect.com/