SCOTRAIL'S crackdown on fare dodgers is likely to hit the earnings of on-board conductors, unions warned yesterday.

Although the train operator predicted significant falls in fare dodging and increases in revenue as a result of introducing automatic ticket machines and barriers at key stations, unions warned that the move could have an adverse effect on conductors.

They currently receive a 5% commission on fares bought on board trains because of the switch to advance sales, and the RMT union is now monitoring the threat to incomes.

It is estimated that ScotRail loses (pounds) 5m a year - about 4% of ticket sales - because of fare dodging. To reduce the problem, it has introduced a gate system, where passengers must pass their tickets through automated barriers before boarding or at the end of their journey.

ScotRail is expected to complete an assessment of their worth next month, when revenue trends will take account of reduced travel over Easter.

However, clear patterns have already emerged, the operator said. There has been ''a huge increase'' in the use of ticket-vending machines; more people are buying season tickets and queuing at ticket centres during peak periods is down.

However, another trend is expected to limit the commission losses to conductors. Passengers boarding at stations without ticket machines have been deliberately seeking out conductors on services, so that they are not delayed at barriers on arrival. Conductors last year received machines that print tickets for use at barriers.

Rising passenger numbers, which stand at 64 million compared to 53 million at privatisation in 1997, could also reduce the impact on conductors, whose basic salaries are (pounds) 19,000 following a pay deal earlier this month.

However, Mike Barry, RMT spokesman, warned that loss of income for conductors could become an issue, although he accepted commission levels were never guaranteed.

''If people have already bought tickets, then clearly there could be a drop in commission,'' he said. ''At the moment, it is not an issue and there is nothing to add other than that we are monitoring the situation.''

Mr Barry acknowledged that automatic ticket gates have led to an increase in jobs and union membership. ScotRail said 100 new staff had been employed for ''revenue protection'' and customer service duties, allowing more ticket checks or help at the barriers.

Similar to those on London Underground, the ticket barriers have been installed at Waverley and Haymarket stations in Edinburgh and Queen Street, Glasgow.

Under a (pounds) 3m investment, about 20 ticket machines and barriers have already been introduced at stations including Polmont, Linlithgow, Lenzie, Hyndland and Anniesland as well as Glasgow Central, where fare dodging is said to cost (pounds) 2m a year.

It appears inevitable that more barriers will be introduced across Strathclyde, the largest commuter network outside London, with research suggesting one in every five passengers in the area fails to buy a ticket before boarding trains.

Further expansion will be a matter for the future ScotRail franchise, Strathclyde Passenger Transport and the Scottish Executive.

The introduction of the barriers has been criticised for causing queuing and increased journey times, but ScotRail insisted yesterday that ''teething problems'' were being resolved as passengers became accustomed to the system.

A spokesman said: ''Experience from south-east England indicates automatic ticket gates make a huge difference in reducing ticketless travel - and we would expect the same thing in Scotland.

''There is no doubt that gates improve revenue collection, but they also reduce congestion by enabling people to pass quicker than they could with manual inspections.''