Commuters are facing disruption as 100 conductors on the London Midland rail service have gone on strike.
Members of the Rail Maritime and Transport (RMT) union walked out for 24 hours at midnight, claiming workers are being pressured to work on Sundays.
The RMT said London Midland, which runs trains between London Euston and Birmingham, has forced managers to cover for conductors on Sundays.
London Midland rejected the union's claim it was bullying managers.
The rail firm said there is a reduced timetable with passengers travelling from Euston to Crewe or Birmingham New Street, having to change at Northampton.
All other London Midland services through Crewe and Birmingham New Street are unaffected.
There is also a reduced service between London Euston, Tring, Milton Keynes and the lines between Bletchley and Bedford, and between Watford Junction and St Albans Abbey.
The RMT's general secretary Bob Crow accused the company of refusing to honour long-standing agreements that former Silverlink staff could opt out of working on Sundays and said the dispute had dragged on for eight years.
"Instead of negotiating sensibly the company seems hell-bent on confrontation and is even intimidating reluctant managers into covering the conductors' safety critical work.
"We believe that is reckless, because it takes three months to train conductors, who must have specialist route knowledge and keep abreast of developments in safety procedures, and these managers have received only a few days' hastily-arranged refresher course."
Safety 'not compromised'
The RMT plans to stage another 24-hour strike next Monday.
London Midland said: "The reason for the dispute is arrangements for Sunday working and calculation of rest-day payments.
"Both of these issues pre-date the start of London Midland's franchise.
"The company has offered to change the calculation and backdate this to the start of the franchise (11 November 2007)."
The company rejected the RMT's allegations it was "bullying" managers and said safety was not being compromised.
The Transport Salaried Staffs Association (TSSA) said it was balloting 30 managers at the company over claims of "undue pressure" to cover for conductors.