13mcg in a breath sample for all Railway Staff working in a safety critical role, includes drivers, conductors, signallers, control staff, technicians ... even the bod on the platform with whistle and lollipop.
Testing is done randomly (quotas per 4 week period for testing) and following any kind of safety incident, failure in 99.9% of cases means instant dismissal and forget getting any sort of job on the railway again.
From experience, most colleagues do not drink for 24 hrs before a shift, I dare some there are some there who will chance it but that's up to them. Although I'm not in an SC role I hold a Personal Track Safety card so could go trackside without notice, plus when travelling for work 'on the cushions' I am expected to assist the train crew in an emergency, therefore I leave the alcohol until Friday or Saturday evenings only unless I'm on call over the weekend when it's none at all.
If you talk to some of the old BR boys boozing used to be endemic until some spectacular accidents in the 60s and 70s. Folklore says that Tommy Bray (Mallards fireman on the world record run) shovelled around 7 tons of coal during the record run and drank an equivalent amount of beer afterwards!