Not too radically different I don't think.
It would be big and well used, since that is the only way you could persuade the powers that be that It was going to be worth the massive public expenditure and wholesale scrapping of the planning laws to get such a thing not just built but in time to reap the benefits (climate, levelling up).
It would be based on existing or near future technology, wholly electrified (opportunity charging), because obvious.
It would still be worthwhile to build a proper high speed network, tunnelling under cities and connecting to airports and parkways/hubs. It would use automation to ensure the network offered genuine one journey route utility, much like the way global air travel is deprecating hub and spoke networks.
It would still be worthwhile to build a slower but very comprehensive regional/commuter network primarily connecting towns with the high speed network at one extreme, remote villages at the other, with nothing really lost by sticking to surface or cut and cover, since quite a lot of destinations and an increasing number of travel hubs are now literally on the edge of towns or off motorway junctions anyway. Advanced in automation and signalling mean you can squeeze so many more trains into those few stations that are going to be in town centres. You would need far less smaller urban stations given personal pods and urban transit will now be the norm for these local trips, using space liberated from the private car. You would need far less trains too, doing more with less, given advances in technology, and because peak time loading and distances are going to continue to fall due to enhanced social mobility, localism and teleworking.
it would be quite ridiculous to expect there was any realistic chance of being able to build anything of significance in the metropolitan or suburban railways sector as it exists today. Even now, and certainly in the future, advances in technology, plus population density and urban space usage would mean any and all available space liberated from the private car, would be given over to rapid transit modes and personal pods. People would think you quite mad for suggesting it could instead be given over to a vehicle that has to travel at least half a mile between stops to be of any use to anyone not seeking to only travel between those stops.
The most radical change would be the opportunity to build an entirely fit for purpose, wholly dedicated freight network, for not just train and wagonload (intermodal), but also parcels/pallets. Perhaps a larger gauge, certainly higher, certainly heavier, it would serve all the major logistics hubs and ports, most of those being in rural areas. Automation and capacity from being a dedicated network with its own lines would ensure slower running speeds still allow for faster and more useful end to end runs than today, which should be enough to be of significant benefit to modern day logistics needs, which would still be dominated by road transport due to parallel advances in technology.
Oh, and of course, you would have ample reason to build extremely long single track passing loop scenic lines, since it would be readily accepted that this would be preferrable for providing access to tourists and exploit the scenery for economic gain, than the alternatives (allowing those views to be enjoyed only by those rich enough to own a private car and pay to use it in a national park).
