This one probably isn't the most pressing need, but much of it is along existing unused right of way, and there isn't any other CTA rail around either.

This leg of the gold line will travel parallel with North Avenue on the currently unused Bloomington viaduct from the River to about Keeler. West of Keeler there it has to share r-o-w with Metra trains, in real life it'll be crowded but there should be a fairly cheap solution. East of the river it'll have a stop under the red and purple lines, then go under North Avenue to LaSalle.