A lot of people seem to be missing the point here.  Bringing Sunday's into the working week isn't that straightforward. You would need to recruit a considerable amount of new staff to enable this to happen otherwise you'd find yourself short at other times during the week.
There was a figure floating around as to how much this would cost the company, I can't recall what it was but I will try to find out, but it was a huge amount.  The company can not afford to do it. 
1.  The cost of additional drivers, train managers & catering staff as mentioned plus,
2.  A payrise for all existing staff to include Sunday's in the working week (for example, Sunday's are worth say ^2000 a year.  ^2000 would need adding to the basic salary)
3.  Sunday is currently overtime and it is therefore not pensionable. Bringing it into the working week would, I think, have bigger financial implications for the company. (I'm not 100% upto speed on pensions, tax etc but I've heard this mentioned!)