Workers on the Heathrow Express rail service are staging the first of two 24-hour strikes on Thursday.
Rail bosses have been criticised for using managers to drive trains during a strike over pay and conditions.
Station staff and other employees, including a few drivers, who belong to the Rail Maritime and Transport union (RMT) will also walk out on Monday.
The Rail Maritime and Transport union (RMT) said lives were being put at risk by using "inadequately-trained managers" to operate the trains.
They voted by 12-1 against a pay and conditions offer.
Heathrow Express said a normal service was running and fully-qualified driver managers were driving trains "as part of their normal duties".
Heathrow Express, which runs services from Paddington to Heathrow Airport, said it was "disappointed" that the union was pressing ahead with strikes.
Up to 80 workers voted 12-1 to walk out on Thursday and Monday.
RMT General secretary Bob Crow, who will join pickets at the firm's depot in Paddington, west London, said: "We have made it clear to Heathrow Express that we are available for talks to resolve this dispute, but our referendum result should tell the company exactly where our members stand.
24-hour walkouts
"The problem remains that Heathrow Express is still expecting our members to pay for a shorter working week with shorter breaks and a squeeze on other conditions.
Heathrow Express, which runs the Paddington to Heathrow Airport service, said only a few of the 275 workforce had failed to report for duties.
"They have also been angered by the company's attempt to dress up the non-consolidated lump sum they are offering instead of the back pay we are entitled to as some sort of concession, and that will not wash. "We remain available to negotiate, but the company will not resolve this dispute by burying their heads in the sand."
Station staff and other staff, including a few drivers, are taking part in two 24-hour periods of industrial action.
Heathrow Express managing director Brian Raven said: "We are confident that we will provide a normal service for our customers and will be the best choice for air passengers travelling to and from Heathrow airport."
RMT general secretary Bob Crow, said: "Our members are out solidly today and if the company keeps their head buried in the sand they will be out solidly on Monday.
"Using inadequately-trained managers to run the trains will not solve this dispute but it will put lives at risk."
