The other two options are related and would piggyback on two unrelated privately funded railway projects west of London that aim to improve rail links with Heathrow Airport.
One, listed as “Heathrow to Langley” would make use of the proposed
Western Rail Link, and could see Elizabeth line trains that currently run between Reading and Paddington divert to go via Heathrow.
The potential for that has already been considered an option and would be less of an extension than making use of an option to create a new loop on the railway, improving rail connections between Heathrow and the west of England.
The other idea being looked at is to make use of another proposed Heathrow railway, the
Southern Rail Link, which aims to improve services to the southwest out towards Woking. In this situation, TfL is looking at running Elizabeth line trains to Staines.
Staines would be an ideal terminus for an extended Elizabeth line as there’s space to expand the station and add the extra platforms that would be necessary, and the station is itself next to a junction which feeds passengers towards Bracknell or to Windsor in the west and Waterloo in central London.
All these options are dependent on funding.
However, an agreement to run Elizabeth line trains over either or both of the proposed privately funded Heathrow railways may help their chances of being built thanks to the extra income from leasing track capacity to more trains, making either of those extensions more likely to be built. Fortunately, when Heathrow Terminal 5’s railway and tube station was being built, they included space for two more mainline train platforms, leaving space for both the Elizabeth line and mainline rail services to use the Heathrow links.