The RTS is the descendant of GMC's entry for the Transbus project which in turn was the descendant of the RTX (Rapid Transit Experimental), an experimental model for which a prototype produced in 1968 with notes of its production dating to early as 1964.
Both the RTX and the Transbus were similar in terms of design to the RTS though had major differences in having a less-rounded body design, a one-step entryway, and (in the case of the Transbus) a 45-foot (13.72 m) length.

Wanting a backup plan in the case that the Transbus project was abandoned, GMC decided to modify the RTX/Transbus design and in 1970 began the project that became the earliest RTS with the first prototype being assembled in 1973 at which point the project went onto hiatus.
Though closer to its predecessors than the production models, the RTS name debuted with this prototype.
After the project was revived in 1974, GMC would later withdraw from the Transbus project and focus their energies on the RTS.
The RTS is the descendant of GMC's entry for the Transbus project which in turn was the descendant of the RTX (Rapid Transit Experimental), an experimental model for which a prototype produced in 1968 with notes of its production dating to early as 1964. Both the RTX and the Transbus were similar in terms of design to the RTS though had major differences in having a less-rounded body design, a one-step entryway, and (in the case of the Transbus) a 45-foot (13.72 m) length.
Wanting a backup plan in the case that the Transbus project was abandoned, GMC decided to modify the RTX/Transbus design and in 1970 began the project that became the earliest RTS with the first prototype being assembled in 1973 at which point the project went onto hiatus. Though closer to its predecessors than the production models, the RTS name debuted with this prototype. After the project was revived in 1974, GMC would later withdraw from the Transbus project and focus their energies on the RTS.
Rapid Transit Series