How do you even prove something like this? Marrakech is 1083.12 km from Zouérat and both are on different railways. In the north, the railways connects to Tunisia but it ends at the Lybian border and it's more than 1400km from Gabes, Tunisia to Sallum, Egypt so the 1083.12 km is the shortest distance. Even the most remote northern railways in Finland and Russia are less than 500km from the next railway. The Panama Canal Railway where I can't figure out what's the closest other station, but there's the Puerto Berrío - Pavas - Medellín line and Puerto Berrío is only 630km away so it's much less.
This of course takes into consideration railway stations you could drive in between. Otherwise, New Zealand will win this, very easily as AFAIK the New Zealand network is a single one with the Interislander ferry and you'd need to cross the 2000km wide Tasman Sea to the next railway in Australia...
Edit: with the question changed and heritage/tourism railroads count, now if we find a single such in New Zealand unconnected then @bregalad 's find is better.