As developments happen near the line (once it is built) they could be asked to fund public transport, to this end a few thousand homes could pay off a couple of million of pounds of the original investment.
This was how it was supposed to be but the local authorities couldn’t agree.
In London there is exactly this mechanism to fund Crossrail 1, called the Mayors Community Infrastructure Levy (MCIL) and it is to be extended for Crossrail 2 (MCIL 2, if it happens). Every new development in London pays a rate per sq metre, with some exceptions for health, education and charitable development. The rate depends on the London Borough; it is highest in central London and some of boroughs with high development values (eg Richmond), and lower in outer boroughs, eg Croydon. The rates will go up for MCIL 2. An average new flat in Outer London will pay about £1500, a large new flat in Chelsea will pay £10k.
The average annual income for the whole of London over the last 3 years is £120m. This is on the back of around 30,000 new homes pa and of course a large amount of commercial property.
For EW Rail, the rates chargeable would be lower because of property value, and in my opinion the rate of building will also be lower. There will also be a need for such a levy or the developer, to fund other transport infrastructure that new development in London doesn’t need to, principally roads (to access the development at all), and bus provision.
All in, I’d expect an EWR equivalent to MCIL to pull in perhaps £30m-£50m pa. That would support borrowing of around £500m-£800m over 30 years or so. But the relevant local authorities would need to agree and commit to the levy for that time, and it may well need primary legislation to enable.