Some may be thinking, but nobody has pointed out on this thread as far as I can see, that there are two effects here.
The first is a general reduction in travel due to the direct effect of Covid (people not wanting to be infected, this affecting trains more than cars) and the indirect effects (many reasons for people to travel don't currently apply). Those will I hope bounce back reasonably quickly whenever we have enough people vaccinated to remove social distancing requirements.
The second is a permanent shift away from working in city centres. This particularly affects the work done in city centre offices, because much of it can be done anywhere. When office use is no longer restricted them some but not all will come back. Many employees prefer the ability to work at home, at least some of the time, and employers have seen that it's possible to maintain productivity even when managers can't see what their staff are doing. City centre retail also falls into the same category, because some of the shift to online purchasing will also be permanent.
The first of these effects can be managed via temporary reduction in services, which has largely happened already. Cost savings are small but people factors such as staff self-isolating and loss of currency in route knowledge make these cuts inevitable. In most cases the trains and other assets are still available and can be brought back to full use when the need arises, although there will be some time lag related to staff training. Over time this will become more difficult as rolling stock leases expire and staff numbers reduce by natural wastage.
The second effect is more significant and largely applies to commuting. There is probably some effect on business travel too, though I have pointed out elsewhere that many businesses already turned a lot of meetings into teleconferences after 2008 so some of this effect may have happened pre-Covid. I'd agree with those that have said the response should be to scale back the peak extra services which determine a lot of the costs of the railway - this applies particularly to London commuting as in many other cities the peak augmentation is minimal. On most lines the off-peak service is a reasonably attractive frequency and I believe usage will recover fairly well, including people who may visit the office for a specific meeting in the middle of the day rather than staying there nine to five.
I'd say it's still too early to tell on the frequent intercity services. Apart from Cross Country most of the fleets concerned are locked in by contract for several years yet, and nobody else is likely to want to lease these fleets so the leasing companies have no incentive to agree to those contracts being terminated. There may be short term measures such as re-deploying bi-mode 80x and mothballing an equivalent number of Voyagers to increase Cross Country capacity and reduce carbon emissions, but I'd suggest we leave it a couple of years before doing anything irreversible.
