The train drivers' union ASLEF's policy is that it can't support a member who has been proved to have been using a moblie phone in the leading cab of a traction unit whilst it's moving. It's even run a campaign warning drivers not to be tempted to use their mobiles because they will lose their job. See the attached picture for a badge they issued....
Texting is a fundamental breech of both the operating rules AND company driving policy (for obvious reasons). Thank goodness nobody was injured.
There's an ORR guidance blurb here....
https://www.orr.gov.uk/media/18562