Pinched part of a seating diagram of a Cross Country Voyager train to show you what the Government's suggested social distancing looks like when applied to a carriage.
Green seats are the only seats that should have passengers. There should be no standing passengers either.
Green seats are the only seats that should have passengers. There should be no standing passengers either.
That makes for a capacity of just 16 passengers in a carriage with 62 seats, or 25.8% of seats. (But of course you also lose 100% of standing capacity at busy periods, which is where the figure of 15% of normal capacity comes from.)
(To be 100% clear: I amended their diagram myself. This isn't something officially put out by Cross Country. They just happened to have a tidy sketch.)
And the other half of the picture presumably looks something like this (please forgive my sub-basement graphics skills)
