"The Pennsy Secondary to Peoria was acquired from PC only after the IT's own Mackinaw River bridge collapsed under the weight of a loaded IT grain train some time around 1975."
Actually, the Pennsy Secondary was acquired on April 1, 1976 and the ITC derailment that destroyed the bridge carrying the original traction mainline over the Mackinaw River took place on August 7, 1977.
Despite ITC's dependence on trackage rights in its last years, the railroad served a number of major industries: ADM's East Plant, West Plant and North Plant in Decatur; Borden Chemical in Illiopolis; Fiat-Allis in Springfield; American Steel Foundries, A. O. Smith, Granite City Steel and Bulk Services Tri-City Port in Granite City; Conoco, Shell and Clark refineries in the Hartford/Roxana/Wood River area; and Owens-Illinois Glass in Alton. The ITC controlled the routing of much of this traffic, especially since it was the sole-serving carrier at a number of them.
ITC had a major interchange with the Rock Island at Peoria (via P&PU) in which each railroad pre-blocked each other's interchange traffic. Some bridge traffic was handled between the Rock Island at Peoria and St. Louis area connections such as the Frisco, Cotton Belt, Southern, L&N, TRRA, Alton & Southern, Katy and MoPac. Well into the 1970's, Madison, IL to East Peoria, IL manifests 200 and 203 had heavy consists of 80 to 100 cars on some days. Much St. Louis-area traffic was routed up to Peoria area interchanges then either east (TP&W), north (C&NW and Rock Island) or west (Burlington and TP&W) from there. They also handled traffic for customers located on the Peoria & Pekin Union Railway - Allied Mills, Caterpillar Tractor Co., Hiram Walker & Sons, Keystone Steel & Wire, etc.
ITC handled "transit" business, i. e., the slow-moving traffic billed to then stored in a holding yard until a broker sells it.
The ITC abandoned its original line into East Peoria on August 1, 1966. TP&W's East Peoria Yard became the road's area terminal. ITC reached it via trackage rights on the Pennsylvania RR's Peoria Secondary from Allentown to Farmdale Jct. and on Norfolk & Western's ex-Nickel Plate line Peoria District from Farmdale Jct. to the N&W-TP&W interchange tracks at "Farmdale." This lasted until December 1969 when ITC extended its trackage rights on the N&W to the P&PU's East Peoria Yard. The ITC's would have used the TP&W's "West Main" to pull Train 200 off the N&W and then back into the TP&W yard and to back out Train 203 then pull it onto the N&W.
Norfolk & Western acquired all ITC from its other owners, gaining control of the railroad on September 1, 1981. On May 8, 1982, the operational merger between the two roads took place and the ITC was no more. The N&W mainline from the TP&W crossing to the junction with P&PU at the Farm Creek Bridge was abandoned in 1983 in favor of trackage rights on the parallel TP&W's West Main.