what is the meaning of singleman dying while riding a train?

He dies traveling: 'on the job' as it were. He dies doing what he loves, in the process of moving from one deal to another. He is as close as a man in his eightes would get to still being 'on the road' as a salesman.

There is also an interesting sub text in that for Singleman being a salesman is about the journey not the destination. Willy is unable to see this perspective which may be partly why he is unsatisfied with his life.