Move along the carriage, now. Make room for the other passengers - and their absurdly overloaded backpacks. With most school and university exams over, the trains of Europe are filling fast. In the next few months, 200,000 travellers aged 25 or below will seek to maximise the mileage they can achieve with their Inter-Rail passes. This absurd annual migration spells trouble for every other European rail traveller - yet there is an obvious, cost-free solution.

Move along the carriage, now. Make room for the other passengers - and their absurdly overloaded backpacks. With most school and university exams over, the trains of Europe are filling fast. In the next few months, 200,000 travellers aged 25 or below will seek to maximise the mileage they can achieve with their Inter-Rail passes. This absurd annual migration spells trouble for every other European rail traveller - yet there is an obvious, cost-free solution.

First, the problem: so many passengers, so little space. Anyone who pays several hundred pounds for an unlimited Continental rail pass reasonably expects to roam widely by train. A few premium services, such as German high-speed ICEs and Thalys links in Belgium and Holland, demand supplements that renders them unattractive to Inter-Railers. But access to the majority of trains is unrestricted: they carry as many passengers as can uncomfortably be squeezed aboard. On popular services between, say, Munich and Vienna, summer in the Inter-City can rapidly become intolerable.

Night trains are worst hit. Because cheap rooms are scarce in alluring cities such as Venice or Cologne, the rational accommodation solution for Inter-Railers is to abandon the city at dusk and colonise an overnight service until dawn. The result: any normal traveller unfortunate enough to join the midnight train to Genoa or Geneva is out of luck. Every compartment is probably already occupied by six Scandinavians. Their underwear will be draped from the luggage racks in a futile attempt to dry the offending items in an atmosphere heavy with undrinkable wine and unsavoury habits, plus the aroma from a forgotten lump of cheese well on its way to becoming a biological hazard.

The woefully misguided idea behind the launch of Inter-Rail in 1972 was - as you may have read in The Independent two summers back - to hit back at the ascent of hitch-hiking. The railway authorities of Europe launched a pass allowing a month's unlimited travel for £32. Soon, the second-class compartments on so-called expresses from Athens to Belgrade and Basel to Nice were bulging with backpackers.

These days hardly anyone hitch-hikes, and a nine-fold fare increase in the price of a Europe-wide Inter-Rail pass has not stifled demand. Indeed, the scheme has been extended - at higher prices - to older travellers.

Nowadays, even in first class, you risk being maimed by a clumsy rucksack-toting passenger. The culprit is the Eurailpass - the brand name for an unlimited travel ticket aimed at travellers who live outside Europe, and which is available in the premium wagon as well as the cheap seats; indeed, some less scrupulous British passengers have procured these passes by buying online and using the address of friends or relatives living in America or Australia in order to comply with the residency qualification.

To ease the train squeeze, Europe's airlines should show some imagination. By offering a standby air pass, they could fill many otherwise empty seats over the next few months. At the best of times, business-focused airlines like British Airways and Lufthansa struggle to sell three out of four seats on the average scheduled flight. When business travellers desert the departure lounges in July and August, the going gets even tougher.

This summer, BA has been flogging Club Europe seats for a fraction of the usual fare in a bid to fill space. Lufthansa has trimmed capacity: on some flights on the key Heathrow-Frankfurt link, the German airline will substitute narrow-bodied jets for wide-bodied Airbuses. But you can bet your bottom euro that both airlines will fly plenty of empty seats around Europe in July and August. They could sell them to standby passengers travelling on air passes.

Here's how. Most people prefer to fly long distances rather than travel by train. So BA or Lufthansa, or indeed Air France-KLM, could offer an unlimited-travel pass for the same basic price as Inter-Rail. The twist is that the passenger would be liable for fees and taxes, adding perhaps £20 to every flight - thereby deterring frivolous journeys.

The result: an immediate switch from rail to air, benefiting airlines and regular train passengers alike. Standby flyers between Paris and Prague might need to change planes at Heathrow, Frankfurt or Amsterdam rather than fly direct. But such a strategy would clear the railways and fill the empty planes.

This scheme would benefit every European Wanderer - a brand name I hereby donate to any airline brave enough to try a standby air pass.

WHEN AMERICAN FLIGHTS OF FANCY WERE THE NORM...

Like most concepts in travel, there is nothing new about standby air passes. Until 1997, two big US airlines, Delta and Northwest, offered a month-long pass for around $399 (£250) - less than the one-way fare from New York to Los Angeles. Any traveller with flexibility could explore America for a pittance.

The deal was simple: at the departure gate for your chosen flight, you registered a desire to travel, then loitered to find out if you were lucky.

In those innocent pre-11 September days, it was easy to be fortunate. A few airline staff appeared to savour the stress they could generate while you waited uncertainly for a seat on the last flight out of Cleveland. But most were conspiratorially co-operative. When a Northwest flight from Quebec City touched down late at Boston, I was allowed to walk across the Tarmac to a nearby plane departing imminently for Charlottetown - a connection time of about two minutes.

Anyone short of cash could cut spending on food and accommodation by shuttling back and forth across the continent; in those days, airline passengers were traditionally served a meal.

Inevitably, the standby system involved an element of risk. An unlimited travel pass placed you well down the batting order when staff came to choose passengers to fill the last few seats. If bad weather cancelled dozens of flights, standby travellers could be stuck for days. But with imagination and perserverance you could, for example, travel from Atlanta to Boston via Cincinnati.

You were also made aware that even once on board you were not guaranteed to travel. If a real, fare-paying passenger turned up at the last minute, you might have to surrender your seat. Yet the serendipity endowed by the air pass easily outweighed the downsides. You could consult the weather charts in Atlanta before choosing between a Delta flight to LA for Disneyland in California and the Orlando departure for Walt Disney World in Florida. And if, on the way to the gate, you passed a flight to somewhere seductive-sounding like Sarasota or San Antonio, there was no penalty for a flighty change of heart.