Austrian rail operator OeBB on Saturday unveiled its new generation of sleeper trains -- a response to demands from travellers for less pollutant alternatives to planes and petrol or diesel cars.

Night trains are starting to make a comeback in Europe thanks to their low-carbon footprint and after years of neglect, operators are now investing heavily to replace their ageing rolling stock.

State-owned OeBB, which operates 20 rail routes across Europe, has been a pioneer in reviving such services.

The company has Europe's largest fleet of sleeper trains and hopes to double the number of its overnight passengers from 1.5 million to three million by 2030.

The new trains feature a modern design, more privacy and more shower facilities.

The first of 33 -- ordered from German conglomerate Siemens in 2018 for a total investment of 720 million euros ($760 million at current value) -- will come into service on December 10.

It will connect the Austrian cities of Vienna and Innsbruck to the German port of Hamburg.

Between then and 2028, it will be phased in on other routes in Austria, Germany, Italy, the Netherlands and Switzerland.

"France will have to wait a bit because this requires new approvals from the national railroad authority," OeBB CEO Andreas Matthae told AFP.

"Unfortunately, we are not yet a unified Europe in the railroad sector."

Environment Minister Leonore Gewessler told AFP: "Every kilometre (mile) by train is a kilometre for climate protection and... a contribution to a better future."

Since 2018 OeBB said all its trains "run on electricity generated exclusively by hydropower, solar and wind power... a milestone for climate protection in Austria"

"I still remember the old sleeping coaches, which were so dark -- the small window, the bunk beds and a curtain," said 69-year-old retiree Rosemarie, who declined to give her full name, as she examined OeBB's sleek new night train on Saturday.

Compared to the old sleeper trains, "this is crazy -- pure luxury", she laughed.

OeBB's Berlin-Paris night train service is to return on December 11, nine years after it was cancelled.