Hong Kong is preparing to open a controversial high speed rail link that promises to slash travel times to mainland China.

After a series of setbacks, including construction delays and a minor derailment during early testing, the line is expected to open in September.

As part of the AU$14.4 billion dollar project, two 26 kilometre-long tunnels have been dug under the densely populated city, to bring the railway to the edge of Hong Kong's famous harbour.

Currently, it takes more than two hours to travel to the neighbouring Chinese metropolis of Guangzhou, but once opened the new line is expected to slash that to just 48 minutes.

9News was given a tour of the new Kowloon terminus, which will be the largest underground high speed railway station in the world.

The CEO of Hong Kong's MTR Corp, Lincoln Leong, proudly showed off the airy, light-filled complex, as workers toiled away on the final fit-out.

"This station is 4 million square feet of real estate, all built underground, but standing here you wouldn't know that we're underground, because of the roof structure we've built," he said.

"All the tracks are underground, from here to the border at Shenzen."

But he conceded the eight-year project had been a challenging exercise.

"Unfortunately, in construction of this particular station we encountered some very difficult ground conditions that have caused some delays," he said.

Those delays were exacerbated when one of the new trains suffered a minor derailment during the early phase of testing.

But while the engineering aspects have been difficult, so too has the politics.

In particular, plans for a co-located immigration checkpoint inside the station, which will see mainland Chinese laws enforced on Hong Kong territory for the first time.

Laws to enable the contentious arrangement passed Hong Kong's legislature during the week.

Dr Sow Keat Tok, a politics expert at the University of Melbourne's Asia Institute, said many in Hong Kong viewed the arrangement as something of a trojan horse.

"They don't know what to expect. If China can do this today, what can they do tomorrow", he said.

The Hong Kong line will connect directly into China's vast high-speed rail network, which measures a staggering 26,000km across the country.

Critics have argued passengers might shun the new express service in favour of existing slower trains because of its higher fares. A ticket from Hong Kong to Guangzhou will cost the equivalent of around A$44.

But the MTR boss is confident the new line will ultimately prove a success.

"It will be fabulous!" he said.