Over 17,000 customers filed complaints about Midland Mainline in spite of a slight increase in the reliability and punctuality of its service. At the same time, complaints about four other rail operators owned by National Express - ScotRail, Central Trains and Silverlink - fell.
Phil White, chief executive, said: "The reason why complaints have risen is that Midland Mainline's management has been actively encouraging people to complain. That's why complaints were up 50 per cent despite Midland Mainline being the most reliable. All our services are better in terms of punctuality and reliability than under BR."
Pre-tax profits at National Express jumped 43 per cent to pounds 35.1m in the six months to June, spurred by a much-awaited pick-up in the number of people using public transport:up to 15 per cent more passengers used its trains.
The bus-to-airports company is also growing by acquisition on both sides of the Atlantic.
It recently spent pounds 12m on Crabtree Harmon, the seventh largest school bus company in the US. It is also preferred bidder for Eurostar, the channel tunnel train service, and Stewart International Airport in New York. More acquisitions are planned.
National Express became the UK's largest rail operator in March when it bought Silverline and Central Trains. It is spending a total of pounds 300m on 113 new trains - more than the Government has demanded as part of its rail franchise agreement.
Shares in the group leapt from 881p to 940p in response to the good news on passenger numbers. Shares in Railtrack, which charges rail operators for access to tracks, also rose to 1485 from 1450.