Just tried metromap and it runs nice on the N900, although it gives an error while installing as metromap.postinst tries to call maemo-select-menu-location (which I guess doesnt exist in maemo 5). This should be pretty easy to fix in the package.

Its not optimized for the screen and doesnt show a full tube map - but gives routes with changes and journey times.

Updated: Seems the 2007 package does show a map (but the UI takes up almost the entire screen leaving very little space for the map). The 2008 package doesnt show a map but has more space to display the route.