With the exception of those of us on this forum, most people travelling by rail are doing so because the car-based alternative (which all things being equal is likely to be cheaper after the sunk cost of buying and maintaining a car) is either unavailable to them, or because it has downsides (traffic, parking scarcity, congestion charges, parking costs, etc).
Apple Mobility Data bears this out, in that while public transport is below car use and car use has been steadily gaining during lockdown, the two modes do nevertheless track each other. As car use further increases, more people will be pushed to rail.
Accordingly, I’m not convinced much needs to be done to make passengers return; once they wish to travel they will. And that goes doubly for “standing room only” services which people didn’t like prior to Covid but will continue to use because anyone using such a service is doing so because the benefits of rail in that context are so great to them that they will endure a less pleasant journey.