group-by is a layer property released in v2.1 of mapnik.
It'll cut down all the repetition, in both layer definitions and the output xml, for the bridges layers. If it works well, it could be expanded to stack all the non-bridge layers properly too.
It's supported (but not documented) in CartoCSS by using the "properties" of a layer definition.
Fixed in 43ff3b2, this issue can be closed.