The victim was identified by a friend as Michael Case, 45, of Homewood. He remains hospitalized Wednesday.
Case is a Navy veteran, husband, father and friend to many.
"He is one of the kindest, sweetest, most gentile that you've ever met in your whole life," said Diana Hildebrand, friend.
"I don't know how it could possibly happen to him. I've never seen the guy have a problem with anybody," said Greg Wedryk, friend.
Case is known for his fun-loving personality, especially at the restaurant where his wife works.
"Our family here at the 5th Quarter has been so effected because we keep them as our own. We think of them as our family," Hildebrand said.
"He's a great man. He knows how to smile and laugh through hard situations, and he's got a great wife to back him up. Wonderful family too," said Jenny Gipson, friend.
The shooter remains in custody but has not been formally identified by police. He is accused of firing a shot through a train window at a conductor on the platform at the Naperville Metra station.
Passengers immediately jumped to detain the gunman while he was still on board. They held him until police arrived to take him into custody. But many still question why he had a gun on board the train in the first place.
"Amtrak does a good job with its resources; it has random security checks, it has a police force, it of course looks at passenger manifests, but you can't just treat it like an airline," said Joe Schweiterman, DePaul University, transportation expert.
Amtrak police forbids passengers from carrying loaded guns onto passenger cars. All firearms must be unloaded and in an approved, locked, hard-sided container. Smaller unloaded firearms must be securely stored within checked baggage. Firearms and ammunition may not be carried in carry-on baggage.
The police would be difficult to enforce, however. There is little security at Amtrak stations. Experts said that's because it would be too expensive and trains are not considered terrorism threats in the same way airplanes are.
"This here appears to be a random act, but it does show just how problems of violence can happen on the rails as well," Schweiterman said.