The limit is the same for all safety critical staff whether you are a driver, shunter, dispatch or signaller etc etc and differs depending how its measured (Breath, Blood or Urine).
The actual limits are...
More than 29 milligrams of alcohol in 100 millilitres of blood, or
More than 13 micrograms of alcohol in 100 millilitres of breath, or
More than 39 milligrams of alcohol in 100 millilitres of urine
Compare this with the road limit...
More than 80 milligrams of alcohol per 100 millilitres of blood, or
More than 35 micrograms of alcohol in 100 millilitres of breath, or
More than 107 milligrams of alcohol per 100 millilitres of urine
All this is set out in Railway Group Standard GE/RT 8070.