Raed Jaser and Chiheb Esseghaier face multiple terrorism-related charges in the alleged plot, which targeted a train travelling from New York to Toronto. Not-guilty pleas have been entered for both men.
The trial has been hearing secret audio recordings of conversations the men had with an undercover FBI agent who gained their trust while posing as a wealthy American businessman with radical views.
At one point, as the trio was on its way to scout a railway bridge outside Toronto, which was to be the target of their alleged plot, Jaser appears to express his doubts to the undercover agent in a moment when Esseghaier isn't around, trial heard.
"The amount of work and the cost involved...it's crazy," Jaser is heard telling the undercover agent in one intercepted conversation.
Later that day in September 2012, the trio arrive at a location outside Toronto, near Lake Ontario and walk along a pair of railway tracks to a bridge, where they examine the tall, metal structure, court heard.
It was at that point, as they knocked on beams, felt the tracks and even picked up a rusty bolt, that Jaser voiced more concerns, trial heard.
"It is a very solid structure, it's very solid," he is heard saying.
"What I am thinking of is, if we do this and that, nothing will happen...there are big pieces of metal....What can we do about it. I mean, it cannot be cut by hand."
Esseghaier, however, is animated at the thought of the plan progressing, and suggests using a high intensity torch to damage the bridge.
"This is very dangerous," he is heard saying. "If the hole is well digged, you cannot slow down, cannot help him."
Three trains are heard going by as the trio surveyed the area, with Jaser pointing out the Via Rail train that was to be at the centre of their alleged plot, court heard.
"This is the one by the way," he is heard saying. "That's the target."
As they walk along the tracks and later survey the area underneath the bridge, Jaser appears to be set on edge by other individuals who are on a hiking trail below the bridge, the trial heard.
He appeared to be particularly worried about Esseghaier, who was talking loudly while flailing his arms in excitement, the undercover officer testified.
"Excuse me, but can you talk without using hand gestures and who knows what...the guy is looking at us man," Jaser is heard saying to Esseghaier at one point, adding repeatedly that he is to speak only in Arabic.
"You are losing on this job, believe me...because I told you the devil sees everything that we can't see."
Esseghaier, meanwhile, did not seem perturbed by the prospect of being observed by others, countering that anyone else couldn't possibly know about their "project," trial heard.
His attitude appeared to irritate Jaser even further, the trial heard, leading Jaser to issue a warning.
"If you make a mistake with something that you don't take seriously...it's a sin," Jaser is heard saying. "If the operation fails...just because of a guy, me for example or you or any person took things lightly, that's a sin."
The alleged plan was to derail a train by targeting a railway bridge in December 2012, court has heard.
Videos of the scouting trip the three men made to the bridge in late September — which were played in court — show a large metal structure, with two railway tracks and a shallow waterway beneath.
Jaser, Esseghaier and the undercover officer are seen walking along the railway tracks to the bridge and are then observed standing on the railway bridge, examining the structure.
Later that day, they headed back to the undercover officer's car where Esseghaier wrote down a list of each man's responsibilities, court heard.
"We need to be organized," Esseghaier is heard saying.
Jaser and Esseghaier were arrested in April 2013.