Someone once told me that the CTA subway tunnels under the river (Red and/or Blue Line) are actually one on top of the other, not next to each other as we would think. The reason for this, I was told, is that it was easier for them to dig deeper rather than wider. Is this true?
Not likely. If you get off of either the Red or Blue line in the loop (Washington or Jackson stops) you have to walk a block east (or west) to get to the other line.
No, they’re side-by-side tubes. The actual river tunnels were built of steel plate in drydock, towed to the proper position over a trench, and flooded to sink into the riverbed. Once anchored, they were pumped dry and connected to the mined tunnels at both ends. This allowed them to avoid steep grades, particularly for the short stretch between the Grand and Washington stations.
I think you’re referring to the Blue Line tunnel under the river at Lake Street. I believe the tubes were indeed stacked, not for ease of construction, but to make provision for a never-built subway under Lake Street that would have diverged from the main route just west of the river. The stub tunnels can still be seen at the curve at that location. The river tubes were stacked to avoid having trains crossing in front of one another at the junction. A mention of this may be seen here:
I can’t find a more detailed discussion, and the fact that Mr. Downtown doesn’t recall it gives me pause, but I’m pretty sure the above is accurate.
The tunnels do not go on top of each other.
There are provisions that were made for extensions that never happened.
If you take the Blue Line station from Grand towards Clark/Lake, look to your right and you’ll see the tunnel open up within 10-20 seconds of leaving the station. This was going to be an extension to allow for operations to connect to what is the Green Line going to the west side.
A similar modification was made south of Roosevelt which was utilized in building a new portal to the Red Line (to Chinatown).
Technically…there are a few tunnels that do actually go UNDER the CTA tunnels. These of course are the old Chicago Freight Tunnels. When the subways were being built, the CFT was forced to move. The CFT tunnels that were being pushed out of the way were forced to go underneath the CTA tunnels. It was done in a crude and haphazard way, but it was done. I have pictures of some of these tunnels where it dips under the CTA tunnels.
Also, when a train does go overhead (and you’re in the CFT), you can hear the train rumbling.
The link Ed posted accurately describes the Lake/Milwaukee arrangement. This is a few hundred feet west of the river, though. The inbound and outbound Blue Line tunnels are still side by side, but at different depths to allow outbound Lake trains to cross under inbound Milwaukee trains.
The arrangement at Roosevelt/State was essentially a tail track that descended to a lower level than the southbound track, originally built to allow connections to a future Archer Avenue subway.
Quite possible I heard a scrambled version of this, or just misremembered. What you describe makes more sense.