The Air Transport Action Group, in collaboration with CFM, Airbus and Boeing, has produced a booklet, "Beginners guide to aviation biofuels" which I thought was a clear introduction to the subject. For more details, see www.enviro.aero/Biofuels.aspx.

While - obviously - it is an aviation industry publication, it does try not to overstate its case. For example, it admits that describing biomass-derived fuel as "carbon neutral" is not strictly true - while carbon absorbed during the growing stage of a plant equals the carbon emitted when the resultant fuel is burnt, there are also carbon emissions during production, harvesting, transportation and creation of the fuel. So biomass for aviation will only lead to an 80% reduction in overall CO2 emissions compared with the equivalent fossil fuels.

The booklet explores things like the technical challenges (the specifications the new fuels need to meet in order to be safe and usable) and the timescales (50% of commercial flights may be bio-fuelled by 2050).

A valuable resource, available from the website or from doddh@atag.org

A good friend of mine criticises the station at Newark Liberty International Station for having platforms numbered 0, 1, 4 and 5. Well, there is a reason for it, but one only obvious to the railway-minded. It is not the most helpful system for passengers to understand!

However, I found one even more confusing at Büchen, in northen Germany. Platforms are numbered 4, 1, 401 and 41 .

Does anyone know of anything odder?

One of the stunning aspects of KLIA Ekspres, the Airport Express between Kuala Lumpur and its international airport, is the VIP service, e-XKL. You have to pre-book and pay a premium fare, but it's well worth it!

As you walk out of immigration at the airport, someone is waiting for you with your name on a board. They take you through Customs and down to the train, and put you in charge of a porter who rides with you to the city. At Sentral station, the porter takes your bag and escorts you up the escalator to the station concourse, where a driver is waiting to show you to a limo: this takes you to your hotel or office. It's wonderful!

One reason it can work as it does, I imagine, is that Malaysia is a relatively low-wage economy. Could such a system work in the higher-paid west? How?

There are bits which could be cut back - the porter escort isn't needed as long as the passenger is properly met at the downtown terminus, for example.

How much would a system cost in Rome, Vienna or London? How much would it need to be used - how many customers would it need - to pay its way? At what price?

These are hard times, financially, but people are showing they appreciate value for money. And now is a good time to be assessing this kind of value-added service for when the upturn comes.

Your ideas would be appreciated.

I've heard a lot about plans for better train services to Madrid's Barajas airport - but what is the status of these plans?

One plan is for a regional link - a fast service between Chamartin and Atocha, the main line stations in the city, and the airport. The existing metro line 12 is good and well connected, but the planned regional link (which I'm told is under construction) would serve a different market.

Another is for a high-speed connection using an eastern bypass to the city, the belt line. Under this, AVE trains between Barcelona and Madrid would serve the airport: they would run alternately Barcelona - Airport - Chamartin - Atocha and Barcelona - Atocha - Chamartin - Airport.

Information would be much appreciated.

You can imagine that, as Director General of IARO, I have a Google Alert set up for IARO - so when we are mentioned on the web, I am told.

Just recently there's been a flood of mentions - but not about us!

A company called International Aerospace Industries seems to have launched recently, and its code is IARO!

You have been warned!

The real one remains where it always has done - look at the IARO website! IARO's main web-site

Someone once said that a sneeze was difficult to define - you knew when it happened, but defining it was difficult!

So it seems sometimes with airport expresses.

There are, I reckon, 16 of these around the world. Why these 16? Well, they all meet criteria I have informally defined to myself and which seem to be generally accepted. They do exclude some trains which are sometimes described as airport expresses: they do admit one which calls itself an airport express but some think isn't.

So - it's a high quality train service between a city and an airport, primarily serving the needs of the airport (so if it stops between city and airport, it does so for airport purposes - primarily for air passengers). It can run beyond the city but again, primarily for airport purposes (and the classic example is Flytoget in Oslo, running to Asker and shortly to Drammen, in the western suburbs of Oslo, but only carrying passengers to and from Gardermoen airport). An airport express is a business class experience, fast, reliable and comfortable, designed for the air passenger. It sometimes charges a premium fare, but always gives value for money.

Is that adequate? What have I missed out?

Russian Railways published some interesting statistics about the usage of its Moscow - Western Europe train services.

Briefly, in their first year of operation, services to 4 destinations carried 27,000 passengers - 1500 on the Paris service (which has a 12 hour stop in Berlin in one direction and a 7 hour stop in the other); 6200 on the Munich service; 7000 on the Basel service and 12000 on the Amsterdam service.

There is much debate about the demand for train and plane on short haul trips: statistics like these show there is alos a demand for the very long train journey too.