Odds are that most of those rushing down Jackson and over the bridge have no concept of the historical significance of the street they are walking down let alone, what lies below street level on the West Loop side of the bridge. In 1926 Jackson Blvd was Route 66, both eastbound and westbound until 1955 when it became one way from Michigan Ave to Ogden Ave. The foot bound commuters are traveling on the first few miles of the the famous route, with no realization that that it had westbound lanes leading all the way to Santa Monica, CA.
Below this portion of Jackson Blvd from Canal to the river bank lay the tracks of Metra and Amtrak feeding into Union Station. The only parts that are visible to street bound travelers are the train sheds, large greenhouse looking corridors, that cover the southbound track. Historic survivors in themselves witnesses to the Alton Limited, and Pennsylvania's T-1, somehow spared the air rights glut of the 70's and 80's. Under these sheds Metra operates two lines with significant connection to the Santa Fe passenger operations that once where.
The most important of these is a line that runs from Union Station to Aurora. For you see this mere 40+ miles of track is the home to Santa Fe's now BNSF's last surviving passenger trains.
View from inside the locomotive, notice the BNSF lettering above the passenger car door, reminiscent of the Santa Fe lettering on silver cars indicative of Santa Fe's Super Chief, and other passenger trains.
BNSF operates these passenger trains for Metra, meaning you won't see any Warbonnet F40PHM's pulling these trains. Instead you will see Metra locomotives pulling them.
These passenger trains are the last operating with any direct connection and lineage to Santa Fe. BNSF takes pride in this as you can see since the lettering on the passenger cars gives a close resemblance to what we would have been seen on both Santa Fe and CB&Q passenger cars of the past. Looking at these passenger cars one is reminded of the high-liners Santa Fe operated on such trains as the El Capitan, but inside they are standard commuter cars like those on the rest of the Metra system.
Metra also operates one other line of significant lineage as well, that is its Heritage Corridor. This line is entirely operated by Metra but leaves from the same southbound tracks as the BNSF operated trains. The Heritage Corridor is aptly named and travels down the same tracks as the famous Santa Fe streamlines did, and also crosses paths with Route 66 a few times. The line runs to the Route 66 town of Joliet.
Historically speaking Santa Fe's trains would leave southbound out of Dearborn Station, which is actually across the Chicago River a mile to the southwest of Union Station, there are no tracks going to Dearborn Station now. Riding on and experiencing these trains is something any fan of Santa Fe's trains need to do, to have some final connection the past.