The fourth main was built between Eola and Downers Grove in the late 20s(‘28 and ‘29) and was removed in ‘32 and ‘33 when the Great Depression hit. Besides evidence in employee timetables and track diagrams there is still one last remnant at Downers Grove between Belmont and Main St. on the north side where a track runs parallel to the mains before swinging a bit northerly into what is left of the Downers Grove yard.
That’s short stretch is the last physical evidence of the fourth main. That is, other than bridge abutments at Salt creek and the Du Page river.
I do have a photo taken between Lisle and Naperville depicting the existence of the fourth in my collection. I think there’s a copy in the members only photo section of the BRHS website.
The elevation plans were resurrected after WWII in great detail(the BRHS archives has a copy ). The railroad and most the suburbs were anxious to proceed. But Berwyn and one other town(which I can’t recall at this moment) refused to agree and the plan died.
Leo Philipp