AtD translation: brick courses inside the little tunnels

[Mike Jing]

P688.15-20   Only thing to do really was to try and take Renfrew by
surprise. On the way up to Cambridge once again, English country, green and
misty, booming past, brick courses inside the little tunnels spinning by in
helical purity, the smell of fens, the distant reach of water-sky
reflecting the German Sea, for the first time in a good while Lew felt the
desolate stomach-spasms of exile, and found himself longing for Chicago, .
. .

What are those "brick courses inside the little tunnels" exactly?