How is Merseyrail separate from the National Rail/former British Rail network, any more than any of the London, Glasgow, Birmingham or Manchester suburban services are? Although it has a different franchising agreement, It's entirely ex British Rail now Network Rail operated and maintained infrastructure. How does it differ from say Finsbury Park to Moorgate, or the London Overground services?
Heathrow Express but no Gatwick or Stansted Express.
Sheffield Supertram but no Edinburgh Trams.
Merseyrail but no London Overground.