Perhaps there are not inter-city routes in the same way as their used to be, but in terms of ticketing and facilities, there are differences between the various services offered in this country. (Potential) passengers can generally tell them apart by the TOC if they are well informed, but this surely isn't the best way of organising the system, not least because service types can vary within TOCs (which indeed has led to the likes of EMR, GA, Scotrail etc bringing back the name 'Intercity' to differentiate within their network, and even causing WMR to split their brand in half, proving that there is at least some value in advertising the kind of service being offered).
It's certainly feasible to imagine a set of criteria that can be applied. The idea is to communicate that the trains in question are comfortable, reasonably fast, long distance and require booking in advance. Perhaps, these days, that would include quite a lot of the network. Or maybe a three-tier system (or four?) like many European countries would be more appropriate: IC, ICE, etc.
I'm reminded of, for example, the
criteria for certification of EuroCity services (albeit no longer the pinnacle of European train travel):
- train through two or more countries
- all cars air-conditioned
- stop only at stations serving major cities
- stops scheduled to last no more than five minutes, in special cases up to 15 minutes
- food and beverages available onboard (preferably from a dining car)
- conductors speak at least two languages, one of which must be English, French, or German
- average speed (including stops) above 90 kilometres per hour (56 mph), exceptions for routes including mountainous terrain and train ferries
A domestic equivalent of this could specify, the requirement/possibility of seat reservations, first class facilities, catering, etc. Again perhaps with multiple tiers to differentiate your Avantis (Intercity or Intercity Express) from your TPEs (Regional Express or Intercity). Surely that would help the average less informed passenger looking to travel from, say, London-Birmingham to know what to expect from each of the option they are presented with? (And, frankly, to warn e.g. unsuspecting XC passengers what not to expect...)