Elizabeth line
Default from every terminal into central London and beyond Paddington. Slower than the Express to Paddington, and the right train when your hotel is not at Paddington.
LHR · Out and back · published · checked
LHR to the city and back
Both airport railways work. The mistake is the Piccadilly line with a suitcase, or a taxi that sits on the M4 while the train leaves without you.
Have permission before you land
British and Irish citizens use their passport. Everyone else who will pass the border needs an ETA, a visa, or another status that already exists. Apply on gov.uk, not on a lookalike site. Allow three working days even though many decisions arrive in minutes.
Airside transit that never reaches passport control is a different, narrower case. The Home Office says that path at Heathrow does not currently need an ETA. The moment you exit, it does.
Pick the train for the terminal you are in
Elizabeth line stations sit at Terminals 2 and 3, at Terminal 4, and at Terminal 5. That is the default into the centre.
Heathrow Express stops at Terminals 2 and 3 and at Terminal 5, then Paddington in 15 minutes from Terminals 2 and 3, a few minutes more from Terminal 5. Terminal 4 changes at Terminals 2 and 3 for a free transfer of about four minutes. Trains are about every 15 minutes. The first Express from Paddington is early morning. Check TfL and the Express timetable on the day. Fares move.
Stay on a station you can name
Paddington, or an Elizabeth line station in the centre. The Tube onward is for one stop, not for a tour of the map.
Contactless pay is the ordinary way. A paper ticket is the slow way.
Back to the right terminal
The return is the same railway, to the terminal on the boarding pass. Three hours before a long-haul. Heathrow is a set of stations, not one door.
If the flight is very early, sleep there. Do not bet the connection on the first train and a security queue.
Heathrow Express
Fifteen minutes from Terminals 2 and 3 to Paddington. Terminal 4 changes. Pay the higher fare only when those minutes matter.
What not to do
Do not take the Piccadilly line with luggage unless the other trains have stopped. Do not take a taxi to the centre in daytime traffic. Do not start for Bath.
Live detail lives on the operator's site: Heathrow, Elizabeth line (opens in a new tab), Heathrow Express (opens in a new tab).