London Midland staff are being balloted for strike action in a dispute about pay and conditions.
Sunday services have already been disrupted by drivers refusing to work overtime in protest at salary differences and leave entitlement.
The firm was formed from two companies, Central and Silverlink, in 2007 but pay and conditions in the new firm remain unequal, the Aslef union said.
London Midland said the train drivers' union should negotiate.
Former Silverlink workers receive six days more leave but are paid up to £1,500 a year less than former Central workers.
About three quarters of Aslef union members voted in favour of strike action on 23 December, but the ballot was declared unlawful.
The service operator sought an injunction based on ballot irregularities, which the High Court upheld.
Aslef said the irregularities had concerned three ballot papers.
It said it would re-ballot its members this week.
Mick Whelan, from Aslef, added: "When it took over in 2007, the company knew they had different terms and conditions for the drivers employed.
'Would be devastating'
"Since then we have been seeking a proper arrangement for the same rate of pay for the same job."
Wallace Weatherill, London Midland's operations and safety director, said: "A strike would be devastating for us and we will do everything we possibly can to stop that happening.
"The trade union needs to come to the table rather than balloting and try to put its demands in a more reasonable position."
The company runs services through London, Hertfordshire, Buckinghamshire, Bedfordshire, Northamptonshire, Warwickshire, Staffordshire, Herefordshire, Worcestershire, the West Midlands, Shropshire, Cheshire and Merseyside.