Apparently part of London's subway network causes problems for kohanim because it is underneath a museum that includes human remains:
Due to the Science Museum and the Underground entrance sharing the same roof, at present, Kohanim are unable to use South Kensington station and any underground line that passes through it.
There is, therefore, currently a proposal to build an extra structure outside the entrance to the station: you can see before-and-after pictures here. I've found an explanation which says:
The proposed structure would be set to left and right of the entrance to the subway, with a partial horizontal return above set just under the heavy existing cornice and forming a small roof structure. It is understood that the intention is symbolically to sever the entrance to the station from the building above, which houses human remains. This would allow access to the station to Kohanim. To be effective in this regard it has to extend beyond the outermost line of the cornice to the roof above, and so projects out from the entrance frontage beyond the elevation of the porch structure which it adjoins, although only on the right hand side.
My question is: how does this 'work' halachically? Looking at the pictures, I can't understand how the partial archway (covering only half of the entrance, ie the right-hand side) would suffice to prevent tumat met from flowing into the subway network. Can anyone figure it out?