But you can't use an interrail pass within capital cities?
Who told you that, or do you mean on the tube and bus?
Eurail is not valid in busses, with the exception of busses run by the railways themselves but you won't find those in cities.
Eurail is valid in trains of participating companies,
including their trains within cities. Elizabeth line, Thameslink and London Overground are valid, as well as all other mainline rail operators except Heathrow Express.
London Underground, Docklands Light Railway, Trams and Buses are not valid
I couldn't use it on the Underground if that's what you mean. My first day using one, took me first Class to London St Pancras, then Eurostar to Paris and then Paris to Bordeaux, all first class.
My wife and Daughter travelled for a week in Switzland last year using just the Swiss 1 week pass on their trains and buses. they could have used the interrail, but it was cheaper just to buy the one week Swiss pass. They had to pay seperate though to travel by train up the Eiger.
https://www.seat61.com/jungfrau.htm