That's a good idea in principle, but don't forget how NR was forced to sell off its income generating arches to pay back the Treasury !
Indeed, however, even if that happened again, the value of the asset would be higher than just a car park or disused land.
That presumes there is flexibility in reality, and compromises value received in the medium term. As
@yorksrob observes, the freedom to retain income generating assets isn't unlimited, while the reality will be that acting as a developer will mean that obligations (and costs) will be retained, but future freedom (especially for residential property) will be limited.
It depends on how the development happens, and that in part would probably depend on having railway development options.
For example, the station has a former goods yard, some is used for parking, the rest is idle. There's the potential for a loop to be built to give a second platform for the station on a single track line. The (say) housing/ flats are built far enough back facilitate this upgrade (which may not ever happen) and set aside a bit of extra land for future car park expansion.
The fact that peak parking for a station is at different times to housing means that if you provide the "correct" numbers for the local planning authority you'd actually be providing more than is needed (as long as some of that parking is flexible, so either residents or railway users can use it).
By having the travel time to the platform ready to board a train being only a couple of minutes more than driving to the car park entrance could also increase the numbers of people locally using the railways.
Also, such a development is likely to attract less objections from the local population (especially if there's improvements at the station, such as more car and cycle parking, more shelters at the station, that they can see that in the future their station could see an upgrade, etc.) when compared to adding the same number of homes nearby.
There's always the risk of short term thinking causing issues, however, that's a reason to ensure that there is at least a medium term plan for flexibility in what is put forwards.
The scale doesn't even need to be large, a few blocks (say 10) which are each a marionette over parking spaces to deliver the equivalent of a single private parking space in addition to the total parking already provided (as a minimum, but ideally more) at a rural station would likely be of a scale that the community wouldn't be too hostile but could easily add 1,000 extra passenger movements a year to that station (two people going to work 200 days year would be 800). Whilst for the majority of stations that's hardly going to change the numbers, for others it could easily be over 10% more passengers (and for several others easily over 5%) in addition to the income from the (I'm assuming) rent from those new buildings.
In a larger locations then it be flats over a decked car park or flats with basement and ground level parking to increase parking numbers whilst gaining an income.
In yet larger places or could be a larger (say 6 storey block) of mixed use, with retail and restaurants at ground level and a mix of flats, offices and maybe a hotel above (most likely split by lift/stair core, so one core serves the hotel, one the offices and another serves the flats, although the offices and hotel could share one core so fewer lifts overall were required for peak demand).
Yes, you're not going to see the sorts of large income from building over a central city station, however there's likely to be far more which could be done and so over time the total could still be a reasonable value.
Start small and prove that it works and the risk of being required to sell it all off in a short term response to something could be reduced, although if it was then a few flats at £100,000+ each is going to be far more than £10,000 for an acre of wasteland, even just getting planning permission would increase that wasteland to being worth at least £100,000, if not closer to £500,000 if that's for something 25 flats and an enlarged car park in a sightly more desirable location.