Are the lanes next to the historical route the original Route 66?

The lanes you may notice to the west of the two-lane historical Route 66 are actually the southbound lanes of the divided highway that made up the Route 66 alignment eventually replaced by Interstate 55. Parts of the divided highway still exist, for example the 15.9 mile long section starting south of Joliet and extending to north of Wilmington (listed in the National Register of Historic Places). However, most of the divided highway was abandoned many years ago, with only the northbound lanes remaining. The southbound lanes are, for the most part, in a state of decay, but often offers a look at the older composition of the highway, which was concrete slabs with control joints; however, much of these lanes have newer asphalt pavement, albeit in poor shape. Most of the bridges on the abandoned lanes are unsafe or missing, with barricades blocking them.
In some cases, the abandoned lanes trail off under I-55, and there are also instances where the former divided highway alignment is fully under the Interstate. You might also notice instances of roadbed heading off to the east or west that go nowhere. Often, these are crossing roads that were closed when the Interstate was constructed.

Some sections of the abandoned lanes have been reclaimed to serve as parks or bicycle/pedestrian paths or for turnouts, such as the one at Cayuga for viewing the Meramec Caverns barn sign. However, most of the lanes are not open for traffic of any kind. Considering the condition, it’s not likely the drive would be pleasant.
As far as being original, some of the divided highway bypassed previous alignments, while some is on top of the original roadbed. The original road, built as Illinois 4, tended to be quite narrow with thin pavement that could not withstand heavy traffic, especially military vehicles, that once traveled Route 66.
Are There Two Different Routes From Springfield To Saint Louis?
There are many locations where there are different alignments of Route 66, some originally going straight through towns along the way, with later alignments bypassing those towns or rerouting to avoid downtown areas, such as the three alignments in Pontiac. However, there is a major alignment difference running from Springfield to south of Staunton, one approximating the current I-55 alignment, the other the original reuse of Illinois Route 4 lying farther west.

The original path of Route 66 in Illinois used Illinois 4 from Chicago to Saint Louis. Over the years, much of Illinois 4 has been renamed, although Illinois 4 still exists from Springfield south. This is the original path of Route 66, offering some of the more interesting aspects of the road. It can be harder to follow this path, but the key is to follow Illinois 4; there are some turnoffs along the way that are marked as historic 66, offering original alignments of Route 66 (and that of Illinois 4) that you will want to take. Some will merely give you a feel of the narrow zig-zagging road that was state-of-the-art in its time, while others offer even more. One of these is a portion paved in brick near Auburn, while another has a small section with turkey tracks frozen in time near Nilwood. This western alignment was only active as Route 66 from 1926 to 1930, when the alignment moved farther east.
It may be worth noting that Illinois 4 was “retired” wherever it coincided with Route 66 in the mid-1930s, which is why this section designated as Illinois 4 still exists today, while the rest has disappeared from maps. Apparently this original routing of Route 66 was only meant to be temporary, with the later eastern alignment intended to be the permanent route once completed. However, with the following of the already Illinois 4, Illinois was the first state to have Route 66 entirely paved, which it was from the very beginning.
Why A Duck?
While there’s no viaduct there, the stories you may have heard about the Marx family having a farm on what would become Route 66 are true. In 1917, Minnie, the boys’ mom, bought a chicken farm at the northeast corner of LaGrange and Joliet Roads, near where a White Castle currently stands. Groucho often talked of how she bought the farm in order to keep her sons from being drafted for WWI, but that is probably just another tall tale from Groucho. Gummo actually served, apparently the only son that was not rejected for one reason or another.

One thing people seem to agree on, though, is that the Marx family was not cut out to be farmers. 200 chickens were purchased, and coops built, with the intent to sell eggs. Groucho recalled that there were so many rats eating eggs they had to go into town to buy eggs to avoid the embarrassment when people came to visit the farm. In the meantime, Zeppo gave the raising of guinea pigs a shot, but nobody bought them and soon the basement was overrun with the furry critters, making it difficult to enter the cellar to pump water.
Groucho stated that they spent more time at Wrigley Field watching Cubs games than they did working at the farm (although it wasn’t actually named Wrigley Field until 1927, years after they left Chicago, still holding the original name of Weeghman Park until 1920). Eventually they gave up and moved to Chicago, with all of them leaving to return to New York City in 1920.