Unlikely to be more than one train at a time between Aberdare and Hitwaun anyway, so "one tram in steam"!
I think line of sight operation gets you more than not having to have block signalling. If the way the Croyton trams operate is permitted for new schemes it would mean fewer restrictions on level crossings (as suggested below) and indeed no need to even fence off the right of way.
Though if it was wired, the rules for 25 kV operation might be somewhat stricter than at tramway voltages.
Just needs relaying plus if operating as a tramway makes it easier regarding the situation with the two level crossings