It must be a racing certainty that this line will be profitable.
Far from a Racing Certainty, the 2018 Document 'The Case for East West Rail'
https://assets.publishing.service.g...or-east-west-rail-western-section-phase-2.pdf has the following projections:

The fact it wasn't easy to find the Business Case on a google search was instructive in the first place!
Given these numbers were calculated pre-COVID (I haven't seen any refreshed numbers published although I'm 99% certain somebody will have refreshed them for the DfT) and the world has changed considerably since then, the lower-end BCR looks rather close to 1.0
I happen to think that it *will* be very successful, as new lines naturally underestimate demand as there's little data to make projections on. But the fact that there's been a lot of 'value-engineering' (cost-cutting) such as dropping Electrification and no bi-di on the project should indicate it was (and is) not a foregone conclusion about being a roaring success. To repeat (for fear anybody accuses me of doom-saying) I think it will be very successful, and that hopefully the modelling used for new lines will be able to incorporate the results into future appraisals.
I wonder if this is the treasury (pays the outgoing) and the department (receives the income) issue that Modern Railways has mentioned a few times.
The outgoings are still allocated to the DfT's budget, so it's in the DfT's interest to start generating the income as fast as possible.
Basically if you want to bring in more money but spend money to do so (but profitably) .. it won't pass the current logic.
Spending money to bring in more money is the basis behind all capital spending. If your statement is correct, nothing would be being built at all.