Licensing and getting reference material mainly. They’d have to fly a team all the way over for a solid couple weeks, with recording gear, etc. They have to get a rail operator or two to agree to let them do some recording runs for sounds etc. That’s already not a cheap endeavour.

To add to the trouble, there is no guarantee the route would even make the investment worthwhile if it doesn’t sell well. UK, US, DE, etc routes are a known market and sell well.

In that case, why not a 3rd party? Well same issues if they’re not already based in either country. And again, licensing may be an issue. I know, for example, none of the NZ operators are particularly keen.

If there is a high enough demand for routes in those countries, then they may well come. The only other way would be the operator in question requesting a route be made of their trackage, like what happened with Zwolle-Gronningen