"It not only enclosed a considerable area of marshy saltings, but it also created, by accident, a new harbour at Porthmadog."
"This meant that the slates which were already being exported on a smaller scale from Blaenau Ffestiniog, could be loaded onto much bigger ocean-going ships inside a sheltered harbour... and of course in 1836 the railway is bringing slates down," he said
The majority of that is in the past tense, with the exception of the final part?
Looking at the way the article is reported, the final part looks as if it has been taken from somewhere else: it seems slightly out of context as it's presented in the article.
I'd hazard a guess that the issue arises either from incorrect translation or transcription - newspapers and even the BBC seem to have a lot of staff who have a pretty poor grip on the art of writing precisely.
It's also possible that the offending clause was correct in its original context - for example:
"Imagine that you're in Porthmadog in the nineteenth century: the port is developing apace and of course in 1836 the railway is bringing slates down..."
Hello again darkness, my old friend...