Question: Derby Racer was the name of how many wooden roller coasters that operated at Revere Beach in Revere, Massachusetts, both coasters were racing roller coasters, with side-by-side track pairs where two coaster trains would race each other around the circuit of the coaster, a roller coaster train is a vehicle made up of two or more cars connected by specialized joints which transports passengers around a roller coaster's circuit?
Sentence (from "Derby Racer"): Both coasters were racing roller coasters, with side-by-side track pairs where two coaster trains would race each other around the circuit of the coaster.
Relevant evidence: No