Media playback is unsupported on your device Media caption How BlaBlaCar created a global transport network with empty car seats

A rail strike is a strange time to realise that your start-up is going to be a success but that was the moment that the founders of BlaBlaCar knew that their company was going to take off.

BlaBlaCar is the French online ride-sharing company that pairs people travelling between cities with drivers who have empty seats in their car.

When it was set up in 2006, founders Nicolas Brusson and Frederic Mazzella found it difficult to persuade customers or investors of the idea.

"People thought we were crazy. At the beginning they would say, 'it's interesting but you're doing hitchhiking online and no-one's going to do that,'" Mr Brusson says.

Image caption BlaBlaCar's global headquarters occupy three floors of the "#Cloud" building in Paris

The popularity and success of BlaBlaCar today is likely to make any investor who turned them down 10 years ago cry into their cornflakes. Mr Brusson confirms to the BBC that the latest round of funding values the company at more than £1.2bn ($1.5bn; €1.4bn).

The company makes its money by taking a percentage of the cost of the journeys taken by its 35 million members in 22 countries. It says four million people use BlaBlaCar every month.

A series of 'fortunate, unfortunate' events

The company's big break came in 2007 when the French train network was shut down by a strike and passengers were looking for alternative ways of travelling.

BlaBlaCar saw a massive boost, not only in its usage but also to its profile as media companies lined up to cover the strike-busting solution.

"It was the first time I realised it was very useful," Mr Brusson says. "After this spike the passenger numbers went down but to a much higher level than before, a lot of people came back."

Image copyright Mila HO Image caption The ash cloud rising from a volcano on Iceland's Eyjafjallajokull glacier grounded planes across Europe in 2010

Today one in five adults in France are members of BlaBlaCar.

"We didn't wait to be successful in one market before moving into another," Mr Brusson says. "If we were going to work in France we could work across Europe."

In 2009 it went into Spain, while a year later an explosion of ash from a volcano on Iceland's Eyjafjallajokull glacier grounded planes across Europe, giving BlaBlaCar the second "fortunate, unfortunate" event, which boosted its popularity again.

A French company doing business in English

The company began to grow across Europe, either by moving in or acquiring competitors.

"We are a French company," says Cyrielle Callot, Blablacar's head of growth, "but our main language is English."

Image caption A screen in the BlaBlaCar office shows journeys being taken by users on the platform across Europe

It has an office in London but the main hub is in the very chic "#Cloud" building in Paris - BlaBlaCar takes up the first three floors, while Facebook is a close neighbour.

These two offices link up a global staff of 550 employees who stretch across Europe from Portugal to Russia, the Ukraine and Turkey, and since 2014 they have expanded into India, Brazil and Mexico.

Some countries have taken off more than others, Russia and India in particular have seen what the company describes as "phenomenal growth", the UK has reportedly seen slower growth.

Would you get into a stranger's car?

One evening in Paris we watched as three female BlaBlaCar passengers got into the cars of drivers they had never met.

"I always choose a driver who has his picture on the site and good reviews from former travellers," says one of the women, Lucienne Nault.

Image caption Lucienne Nault is a regular BlaBlaCar user

Ms Nault had just been dropped off in Paris having travelled from Brussels, and was waiting for her second BlaBlaCar driver to pick her up on her way home to the Loire Valley.

"I make sure when I travel there are always other passengers," she says. "After all, in a train one sits next to perfect strangers."

Jessica Ekholm, research director for Gartner, the information technology research company, sees the trust issue as one of the company's biggest ongoing challenges.

"Trust is paramount," she says. "You need to know who's driving you. Is this a good driver? They have their scoring, but can you really trust it? Can you trust the people who have recommended them? Who else is going to be travelling in the car?"

"We collect a lot of information," says BlaBla Car's Cyrielle Callot in response.

"Name, age, gender, profession. More and more we ask people to Facebook Connect. We have peer-to-peer ratings, so whenever you're going to be using Blablacar as a driver or a passenger, each person is going to rate the other. And we have a feature called Ladies Only, so that they can choose rides where only women are travelling."

Not Uber

BlaBlaCar hopes to avoid many of the issues faced by the ride-hailing app Uber. Drivers on the BlaBlaCar platform can only charge for the cost of the journey.

Image caption Nicolas Brusson, co-founder of BlaBlaCar, says the company sticks closer to the ethos of the "sharing economy" than Uber or Airbnb

They do not have to change their insurance or pay tax on the money they receive from passengers as they are technically not making a profit.

Mr Brusson says that companies like Uber and Airbnb have strayed from the original ethos of the sharing economy, as they are offering a business service to consumers, not a sharing service between one consumer and another.

He claims that the "global transport network" BlaBlaCar has built has been based on the insight that the average car occupancy in most countries in the world is around 1.5 seats per car.

"The biggest mode of travel all over Europe and all over the world is the car, " he says. "When you think of the volume of the available seats you have in cars and the fact that they're not occupied, it's absurd, it's like running an airline company with your aircraft three-quarters empty."