The legal test, as quoted by Island in their post:
These include that the loss must be "of a kind which the defendant, when he made the contract, ought to have realised was not unlikely to result from the breach … the words 'not unlikely' … denoting a degree of probability considerably less than an even chance but nevertheless not very unusual and easily foreseeable".
The test is not that the defendant (in this case, Thameslink) must have known about the circumstances in advance of making the contract. The test, as bolded by me, is that the defendant ought to have realised that the circumstances were "not unlikely" to occur as a result of the breach, that the circumstances would be "easily foreseeable."
A 45-minute delay causing someone to be late for collecting their child from daycare is, in my opinion, easily foreseeable. As are the financial consequences of that- I've never experienced a childcare provider who doesn't charge a late collection fee.
I am genuinely interested in why anyone would think the opposite. "Oh we couldn't possibly have foreseen that someone using a commuter railway had a deadline by which time they had to collect their child from daycare" seems, to me, to be an interesting argument for a TOC to try and make.
As for it being open-ended, no, the legal test is that it has to be reasonably foreseeable. That will, of course, depend on the exact circumstances.