I'm sorry but traffic jams predated railway closures. When people had the opportunity to travel by train to avoid the jams they still drove.
And the only reason we have the luxury to have this kind of conversation at all is that increasingly that choice is not being made to the same extent as it has been in the past. Young people delay buying their first car, as they delay buying their first house very largely because those of us who are older have accreted the wealth to ourselves and built a huge imbalance of wealth and property between the generations. Younger people also are slightly more likely to make choices that disadvantage themselves so that others, or the environment, or the planet: can benefit and thrive. Things have changed, I hope for the better and for the long term. In these new circumstances, we should in this thread somehow be trying to find ways to make it possible for people to travel in less selfish, polluting and unsustainable ways. My hope is that people in this forum can find ways to suggest reopening railways in a low-cost and sustainable way even into relatively rural areas. Meanwhile, because of the traffic that the branches are building and will build, we will need to invest more in capacity on the trunk routes.
I awoke to the potential of railways only after the bulk of the Beeching-era closures had gone through. Some of those closures but by no means all must now be occasions, not for regret, but for finding ways to restore or renew them. I look forward to your positive suggestions.
I live in France, an even more car-dominated country than the UK but one where local and regional politicians are pushing hard both for new high-speed railways and selective investment in and reopening of relatively lightly-trafficked rail routes. The political setup is different, and there is much more real devolution to the regions and to the communities of communes. But things are changing fast in Britain too; a Tory Chancellor offers significant new transport funding to combined authorities which are often Labour-dominated! Kind of good, isn't it?