Simon Calder: Changing trains à grande vitesse

The man who pays his way

On your marks: arriving at Paris Nord

Breakfast in London, lunch in Lyon, without leaving the ground – how does that sound? Eurostar – whose new timetable begins tomorrow– promises that on any day from Monday to Friday you can reach the gastronomic hub of France by noon.

Agreed, breakfast will comprise only a coffee and a croissant in the Eurostar departure "lounge" at St Pancras; the train to and through the Channel Tunnel leaves London at 5.40am.

A sharp appetite is just what you will need for the promised arrival in Lyon two minutes before noon – along with a good pair of running shoes.

Eurostar's first departure of the day is due to arrive in Paris at 9.17am. The connecting Train à Grande Vitesse (TGV) departs at 9.58am. What could possibly go wrong? Well, the platform you arrive at, Gare du Nord, is more than three miles from the departure quai at Gare de Lyon.

The train operator is adamant that: "The journey from Paris Gare du Nord to Gare de Lyon usually takes around 30 minutes." But Daniel Elkan, founder of the pro-rail website Snowcarbon.co.uk, says: "Travellers are unknowingly booking connections that are too tight and missing them. To do it comfortably you need at least an hour. For newcomers to European train travel, allowing enough time is particularly important."

The only way to find out who is right was to test the concept. So, for a dry run on a wet Monday morning, I caught the 5.40am from St Pancras, aiming to reach the TGV platforms at Gare de Lyon in time for the notional 9.58. Could I achieve a cross-Paris dash in 41 minutes?

It did not begin well. The first train of the day is allowed a relaxed 2 hours 37 minutes to reach Paris, but it still arrived 10 minutes late. That cut nearly a quarter from my allotted transfer time. Still, Eurostar reckons half an hour is perfectly feasible – and I had done everything to maximise my chances. I had used Eurostar's useful (and free) online seat-selection facility to book the carriage at the front of the train. That meant I would not get entangled with the passengers from the other 17 carriages. I carried only a small backpack. And I had the benefit of previous experience of how Parisian transport works.

A taxi? Even if you manage to reach the head of the queue within, say, 10 minutes, traffic in central Paris can be worse than London (perhaps a congestion charge would help). The odds are better taking the RER (suburban express) line D between Gare du Nord and Gare de Lyon. But first you need a ticket – and an understanding that, to travel on the RER (which isn't a Metro train) within the city, you need a Metro ticket. Eurostar sells them on board the train from London, but only in batches of 10 and higher-than-station prices. So, at 9.30am I joined the queue in the Metro station.

Five minutes and €1.70 later, I was on my way to platform 44 – which is as far from platform 4, where my Eurostar train arrived, as it sounds. Another five minutes ebbed away before a packed commuter train arrived, with a couple more minutes lost in the jostle of crowds getting on and off. The train journey, with a stop at Châtelet, consumed the remaining 11 minutes. And by the time I reached the TGV platforms, it was 10.03 and the train was already perhaps 10 miles south of the station and travelling at 186mph. Had I been I a real Lyon-bound passenger, I would have spent much of the next hour queuing to get my ticket amended for the next departure, and rearranging lunch.

You need friends to win at 'Gare Wars'

Even with a 10-minute train delay from London, making the connection can be done. You need a good sense of direction, confidence as a city cyclist and either a folding bike or two friends in Paris to help you out. One ami waits outside Gare du Nord with a Vélib cycle (the city-run bicycle scheme from which Boris Johnson took his London bike idea), while the second stands by at Gare de Lyon, ready to take your bike and "stable" it – always a problem at Parisian railway stations.

Eurostar still insists that "40 minutes is a generous connection time between Gare du Nord and Gare de Lyon". Mark Smith, the international rail guru known as "the Man in Seat 61", disagrees. "Eurostar is setting people up for failure," he says. "While nine out of 10 Eurostars are on time or within 15 minutes, better than most airlines, this still means one in 10 will be more than 15 minutes late and miss this connection. Frankly, I think the connection will fail on more like one in four occasions. Even if you make it, out of breath, you'll have been stressing all the way. They haven't thought through connections from a real (as opposed to armchair) perspective, with the necessary room for delay."

You can book a safer connection – but only by phone. Eurostar says: "We've designed our online booking engines to search for the best connection times. We advise passengers who wish for a longer connection time to call our contact centre." Welcome to 21st-century rail travel.

A bridge too far?

A more certain way to reach Lyon by lunchtime is to catch one of the remaining direct Eurostar trains (each Saturday from today to 29 June). It's a trial to gauge demand for a more permanent service. From July the only summer journey beyond Paris, Disneyland or Brussels reverts to the usual Avignon run. Except for the key Saturday, mid-vacances: on 17 August, Brits bound for the beautiful south of France must change trains – with a 50-minute sprint across Paris.