Testifying for a third week, an undercover FBI agent said Chiheb Esseghaier pondered the possibility of getting the volcano in Yellowstone National Park to erupt.
Esseghaier, who is charged with various terror-related offences along with co-accused Raed Jaser, spoke of the "death and destruction" a blast could cause, court heard.
"Wouldn't it be great if my enemies' worst national disaster could happen?" the agent cited Esseghaier, 30, as saying.
The Montreal PhD student from Tunisia was "very passionate" about the volcano in the park, which straddles the borders of Wyoming, Idaho and Montana, the agent said.
Ultimately, however, he decided the idea was not feasible.
"No. We cannot do anything to make the volcano erupt. It's very deep. I don't have any access," Esseghaier apparently told the agent.
"It was his conclusion that we need to focus on the project or projects," the agent told Superior Court.
"The train plot was a better bet than the volcano," defence lawyer John Norris, who represents Jaser, said by way of clarification.
The idea of a train attack, apparently, had come from "The Responsible One," a man Esseghaier had met in Iran who supposedly had close ties to top al-Qaida leaders.
According to the testimony, the aim was to cut out about five or six metres of track — possibly using a military grade laser — to derail a Via Rail passenger train as it made its way from New York City to Toronto at about 7 p.m. The FBI agent offered up construction equipment and uniforms in support of the train plot.
Jurors heard the plotters scouted suitable sites to damage the rails, including a bridge in east Toronto. Norris, however, noted the New York train actually enters Toronto from the west.
Another plot the men kicked about — which Norris said had "some pretty obvious problems" — involved recruiting a Muslim cook to poison soldiers on a military base.
Esseghaier even talked to a cook he knew about the idea, but the man stopped taking his calls, court heard.
The agent conceded the poisoning plot was a "little fuzzy," saying it wasn't clear whether Canadian or American soldiers were the target.
Jaser, 35, a Palestinian dispatcher from Markham, Ont., worried about the difficulties of attacking a train. Instead, he apparently thought sniper attacks on high-profile targets was a better idea, court heard. Esseghaier considered him all talk and no action.
Ultimately, Jaser dropped out of the alleged conspiracy in September 2012 because he fretted Esseghaier's poor operational security would get them jailed, the agent testified.
Police arrested both men in April 2013.
Not-guilty pleas have been entered for both Esseghaier, who is self-represented, and Jaser.
Cross-examination of the agent continues Wednesday.