Rail depot in latest release (1.17) of pak128.Britain contains a write square. It might be caused by grahics of a vehicle, probably Metropolitan Bogie Third.
I just checked, and have the same. Also, this exact same vehicle can for me also be found as the white square twice; also at the complete end of the list.
I think that this must be an error in the .dat files which have not been updated to go with the new graphics for these carriages (which have a different layout from the old graphics files, with the actual graphics at row 1, row 0 being filled with a white banner with some identifying text) that I produced about two years ago.
Thanks - there were duplicate definitions of some of these vehicles in Standard - now fixed.