Prior to the mid 1970s or so, chemical freight flows were largely confined to individual cars or cuts of cars in the general merchandise trains that formed the vast bulk of the non-mineral freight flows at that time.
In the 1990s and 2000s, there were several flows of chemicals that justified the use of unit trains of chemical cars, either covered hoppers for bulk solids or tank cars for bulk liquids. Most of these flows were between the Chemical Coast of Texas/Louisiana and the (smaller) Chemical Coast of New Jersey.
The Union Pacific portion of these traffic flows starts on the Chemical Coast in the Houston area, using the directional trackage of the northbound Texas Eagle Route (from/to Palestine) and the southbound Cotton Belt (from/to Corsicana) north into Illinois, where, since the UP/SP merger, all such trains take the former C&EI line to its connection with the (former) Conrail routing east from St. Louis at St. Elmo, IL. (Prior to the merger, SP trains on the Cotton Belt line would connect with Conrail in East St. Louis.)
From East St. Louis or St. Elmo, chemical trains head east to Indianapolis on the (now CSX) St. Louis Line, then onwards on the (now CSX) Indianapolis Line. In Conrail days, chemical trains might take either route at Crestline, using either the former PRR route through Pittsburgh and Harrisburg, or the former New York Central route through Cleveland and Buffalo to Selkirk, and then down the West Shore Line, in both cases, reaching the same points on the New Jersey Chemical Coast. Since the split of Conrail into CSX and NS segments, the former routing is now NS, and CSX tends to keep the chemical trains to itself on the routing through Selkirk.