I’ve been involved in several restorations of passenger service in the UK, working for the national rail administration. A new rail service will provide people along the route with an alternative to driving (and not all can drive) and can also renew confidence in communities helping bring investment, new residents, and greater vitality. It all comes down to testing the markets, involving local stakeholders, understanding system requirements and interfaces, and finding the capital and revenue support! This isn’t easy, but the expertise exists.
New modern rolling stock will be more reliable, less expensive to run, able to run faster than loco-hauled classic stock, and (if the worst happens) more crashworthy. A combination of greater speed, suitable interiors and onboard facilities, and lower costs might just make the difference between a service being viable and not. I’d try to borrow a modern unit from a manufacturer for a proof of concept operation rather than start off with heritage stock. The new customer base will want to test a modern service, not experience a museum-type ambiance.
The big differences in the UK is that we run a passenger railway with some freight (not the other way around) and that the infrastructure is state-owned (putting prioritisation of capacity use in one place albeit freight companies don’t always agree with those choices). Albeit I did work on one scheme to restore a passsnger service to a freight line eighty years after it ceased and one challenge was introducing some discipline into “one engine in steam” freight operations which would need to adhere to a timetable.