The word "comrades" has been written on the negative after the photo was taken. These buses of the 51st Coy MT ASC were used in Brighton to carry Indian wounded to and from their hospital in the Royal Pavilion. The London Transport Museum has a couple of other photos of them.
The buses used early in the war had glass large windows but these were easily damaged by the troops packs or nearby shell fire. The broken glass was so dangerous that the windows were removed and replaced by wooden boards. These were not very weatherproof and in at least one bus the drivers pasted newspapers on the inside to seal up the gaps in the boards.
I now hope someone will make seated WWI soldiers.