Narrow Margin - Peter Hyams

A woman secretly witnesses the murder of her blind date for the evening by a top Mafia boss. She immediately goes into hiding without informing the authorities. When they finally catch up with her, she is unwilling to testify to what she has seen, but the Mafia are on her trail. Accompanied by a deputy district attorney, the woman boards a train travelling through a remote part of Canada. The Mafia know him but they have never seen her.