About 5.5% margin on sales (tesco), but yes as a supermarket buys and sells several times in a year it produces a very different return on capital employed (14%).
For example:
Buy crate of beer for 10
Sell crate of beer for 10.50
5% profit margin on sales, 5% return on capital employed
but as it has huge volumes
Buy crate of beer for 10
Sell crate of beer for 10.50
Buy crate of beer for 10
Sell crate of beer for 10.50
Buy crate of beer for 10
Sell crate of beer for 10.50
Still a 5% profit margin on sales, but now 15% return on capital employed.
However a Rail Franchise isnt the same, you dont make a return on each service operated. You lease the stock for 5 years, employ the staff for a year, use up fuel, have fixed overheads. Theres also the bond you give to the Dft in case of failure, insurance, etc. You operate a fixed number of services, with only a tiny bit of flexibility for special services or line closures. You can put on a train, take the profits and use them to put on two trains next time.
The result is that your capital is effectivley tied up for years on end, you cant just reinvest it in new stock, you also return all the assets you purchased and face depreciation costs as infrastructure, stations, uniforms wear out.
Anyway my point is they are far far from comparable, TOCs capital is upfront and locked away for decades.
Edit: Looking at Virgin Groups figures it has around £33m capital invested and made £65m profit in 2010. Meaning a 197% Return On Capital employed for that year on paper, but it cant access that capital for thirteen years, factor in inflation and half that capitals value has been eroded before it returns. You also have to factor in risk, If a supermarket goes belly up its stock still has value, if a rail franchise goes belly up it has no assets and the Dft takes the performance bond in compensation. You also have the risk of making a loss as revenue is outstripped by costs (very likely on a small margin). The Government has set the Performance bond for the next WCML franchise as £50m, meaning that £50m locked away before the costs the Toc will sink into the franchise itself (management, branding, uniforms, equipment, etc).