It was the wrong type, it said: dry and powdery, not the claggy wet stuff it had expected. To the icicled ears of commuters it sounded pathetic; they were - are - entitled to snort in disbelief.

'It was different snow, but not an awful lot different,' says the Met Office. Because it fell when temperatures were lower than normal - well below freezing - it was drier, harder, more powdery than usual. It came in smaller flakes, which tended to remain separate. Instead of caking together, they were whipped into the air by the icy blast.

But the scale of havoc the snow wreaked on the railway system seemed out of all proportion to its depth and difference. Other countries cope, ran the bitter traveller's lament, so why can't we? The answer, according to Gordon Wiseman of the Railway Gazette, is first to do with equipment, or lack of it, and second to do with the economic strictures British Rail has adopted.

Some stoppages were caused by frozen points or pantographs but most were the result of two main mechanical problems: frozen doors and burned-out motors.

The door problems are well known and are caused by a design clanger.

According to Larry Shore, the director of traction and rolling stock for Network SouthEast - which covers services within a 50-mile radius of London -drain holes in the sliding-door grooves do not work, so water from the snowy boots of commuters soon forms an icy impediment. Modifications are being considered; meanwhile, teams of cleaners are positioned at strategic stations to spray anti-freeze on the thresholds.

But it was the motor problem which was new, and which caused British Rail to add powdery snow to its leaves-on-the-line file of assorted excuses. The scale of the problem is still unclear but, since most of the rolling stock is powered by the kind of motor that can be adversely affected, it is likely to have been a major culprit of the chaos. Shore says the system has lost 35 per cent of its 7,000 units, most suffering traction motor failure.

Most commuter services are driven by traction motors mounted under the trains, taking power either from third rails or overhead cables. Traction motors, most of which are made either by GEC Alsthom of Manchester or Brush Traction of Loughborough, use a lot of power and generate a lot of heat, so they need cooling. The powerful rush of air caused by a moving train is ideal for the job, so the motors incorporate air-intakes to direct cooling blasts into the innards. In a motor where 750 volts may be crackling around, it is desirable that this air should be dry.

Normally it is, even when it rains - rain is heavier than air and is thrust naturally out of the way of the rushing motor. Powdery snow, floating in the air, is different. It enters the motor, melts instantly and causes short-circuiting; the motor must be switched off and repaired or, in extreme circumstances, replaced.

Diesel locomotives are not badly affected by the problem, because air to cool their motors is ducted from inside. It is a heavy irony, says Wiseman, that the ancient rolling stock that was so comprehensively criticised after the Cannon Street crash is able to withstand cold-weather problems.

One way to keep out most snow would be to add a fine filter at the air-intake. But traction motors are expensive enough - between pounds 12,000 and pounds 20,000 each - without added filters, which would only be of use once in a powdery snow.

'Traction motors work well 99.9 per cent of the time,' says Brush Traction.

'And British Rail has found that, if it does put filters on, they don't get maintained as they should.' Instead, some British Rail managers refused to send even operable trains out, wary of the cost of retrieval and repair.

Shore says his motors do have filters, but the mesh is too large to keep most snow out.

The logistics of curing a traction motor problem are horrific, Shore says.

First the unit has to be brought back to a depot, 'using anything we can find'. Then the motors have to be dried out - Network SouthEast has hired industrial air-blowers to do the job. Only then can the extent of the damage be judged.

Network SouthEast has closed several depots in recent years, so the load on the remaining 21 is greater. Once traction motors were sent to BR's repair shop at Eastleigh, but now they must be sent to the private firm that won the repair contract. It is in Glasgow, so it takes days. While units are out of action, there is no alternative but to cut services.

'We're under heavy commercial pressure,' says Shore. 'There's no fat. The Continentals have the same problems, but they have spare rolling stock. The way we have become efficient is to take all the contingencies out. Every unit we have we use.'

Most Continental trains do not have the same powdery snow problem. Air ducts for traction motors are on the side or top of the carriage, set in a 'settling chamber' where the air is stilled and snow settles. That requires ducting inside, diminishing carriage space.

So why don't we have settling chambers? 'In Paris, the railway has a 200 per cent subsidy,' says Shore. 'Ours is 20 per cent, so we have to try to get the maximum number of seats in the space.' But he admits that the bill for the powdery snow crisis is going to be millions - including 400 or more motor repairs, at up to pounds 6,000 each.

Wouldn't it be cheaper to have ducts and filters in the first place?

'Perhaps, but the alternative to having trains with cooling under the floor is often not to get the train at all. 'You should hear the debates at the ministry. They say 'you'll be able to spend pounds 2.6 million and you must get 326 seats'. If you get only 325, you don't get the money.'

Roger Ford, technical editor of Modern Railways, thinks a solution would have been to fit foam filters over the air intakes, discarding them when the snow disappeared. It is the kind of simple solution a British Rail engineering shop might have come up with in the diddly-dum days; now, spending money to save money is not in fashion. And more snow is on the way; may it be wet.