Tom (played by David Meunier), no surname given, was the best man when James and Stephanie Dobbs got married, and is Mcnamara Engineering's general counsel. He's on the train with Stephanie and James, who are now divorced, and schmoozing such parties as the Undersecretary for Mass Transit, trying to smooth over $62 billion of public investment in the new high-speed train.
Tom has serious doubts about the goon (Chance) hanging around Steph which are more than reciprocated, but when she reassures him that this is her protection, he goes without making a fuss. In the meantime he has provided vital intel, informing Steph and Chance that someone pulled all the emergency brakes in an adjacent carriage.
After the hired assassin reveals himself, Steph has to persuade the passengers that their lives depend on getting off the train before it stops - forever. Tom points out they're doing 220 mph, making getting off not an option. Once convinced of the need to uncouple sections of the train, Tom wants to be the one who stays behind to do it, and send Steph to safety. His look as he leaves says his feelings are more than 'best friend's wife'. How that may have worked out, after James was shown to be behind the plot to kill her, is not revealed. ("Pilot")