By Simon Montague BBC transport correspondent

Rail companies are warning that orders for new passenger trains are going to dramatically fall, undermining the government's target for increasing rail use.

Orders for new rolling stock are set to tumble to 100 new carriages a year, compared with the current level of 1,000 passenger carriages annually.

The industry says the reason is that train operators cannot squeeze any more services into already overcrowded timetables.

The Strategic Rail Authority has already warned that the government's target of a 50% increase in passengers by 2010 will not be met.

The rolling stock industry now believes that even the SRA's latest forecast of 35% growth cannot be achieved.

Delays in upgrading power lines mean new stock cannot be used

Senior managers forecast orders for 800 new carriages over the next eight years, compared with orders for 4,000 carriages since privatisation.

Future orders will mostly be replacement trains for worn out rolling stock, with few new carriages to provide extra passenger capacity.

Rolling stock bosses have also confirmed that hundreds of carriages already ordered will have to be mothballed in Ministry of Defence depots, because of delays in upgrading the power supply to run them.

Two-year delay

South West Trains is taking delivery of 785 carriages which need extra current to drive more powerful engines and air-conditioning systems.

The current "third rail" power supply can only handle 150 of the new units.

Many passengers may have to put with ancient "slam door" trains until 2006, an extra two years, until Network Rail completes a long delayed upgrade of trackside electricity in southern England.

The problem also affects passengers on South Central and Connex South East, who face similar delays in replacing worn out 1960s rolling stock with faster, sliding door trains.

Once current orders are in service, the rolling stock industry says Britain will have the youngest train fleet in Europe, with the 12,000 passenger carriages having an average age of 16 years.