Andrew writes
I was always under the impression that you are entitled to 15 minutes
break after working four hours, and legally can not be forced to work
longer than four hours without a break.

That's not the case.

As stated elsewhere, if you work more than six hours, you are entitled
to a break of 20mins. For the majority of workers, that means the lunch
break - there is no legal entitlement to any additional tea-break. Of
course contractual arrangements may allow for more generous breaks.

http://www.legislation.hmso.gov.uk/s...9981833.htm#12
http://www.dti.gov.uk/er/work_time_r...6.htm#section6

This isn't specifically railway work, but almost every job I've done
has adhered to this.

You've had some good employers. Many people have the legal minimum and
no more.

--
Dave