The M4 corridor is Heathrow's western catchment, and since the Elizabeth line opened there is a genuine rail option from some of it. So which wins — the train or the car? It depends on your town, your bags and your flight time.
The Elizabeth line, honestly
From Reading and Maidenhead the Elizabeth line reaches Heathrow with a change, and from Slough it is quick — but it still drops you at a Heathrow station, not your terminal door, and you carry your bags through the change. Windsor has no through service at all; it means a change at Slough.
The M4 car, honestly
One leg, door to terminal, any hour. From Slough or Windsor it is 15–20 minutes; from Reading about 40–55. For a group or a family the fixed fare beats several rail tickets and there is no change to manage. Full fares: Berkshire & Thames Valley to Heathrow.
The decider is usually the flight time
Heathrow's first departures leave around 6am. The Elizabeth line does not run early enough to suit a 4:30am check-in, so an early flight is a car job by default. For a relaxed midday flight, solo, light, from a town on the line — the train is fine and cheap.
Heading to Windsor itself rather than the airport? Our London to Windsor Castle guide covers the tourist run. And see the Berkshire taxi hub for the whole county.