It shouldn't be about employing more people on the railway to operate the existing service, that wouldn't be a good use of resources.
But building new infrastructure, for example HS2, East-West Rail etc would be an investment for the country, create jobs in construction and the supply chain and leave a legacy for decades to come that will make money for the country and pay back the investment.
Agreed - passenger numbers will take a while to recover , and I can see a reduction in the timetable/ resource plans on many routes . which will require less trains in service / and everything that goes with that. Hopefully there will be a recovery in numbers , but not necessarily to recent patterns in operation.
The 1920's and 1930's saw a great amount of "public works" which in many areas , apart from the National Grid , school building (think how many schools you went to were new build in various "art-deco" styles ?) a stimulation of the private housing boom through cheap money and a benign planning regime , arterial roads and so on. Even cinema building assisted the Northern steel industries !
Transport wise you had the suburban Tube extensions , large station schemes such as Leeds (new) , Cardiff , Newton Abbot , Swansea High St , smaller stations such as South Kenton , Apsley etc , and large (for the time) power signalling schemes. Most pump primed by tax repeals , loans and other incentives.