The rules of probability, described below, will allow you to calculate this value. Briefly, I need to know the fraction of stations that witness such an event each year, the fraction of a year that the 15 minutes following the event represents, and the total time I was in a position to witness such an event (i.e., I was in the subway.) These fractions are then multiplied (see Rule 2 below):
[(10 events/year)x(1/400 stations)]x
[(0.25hr) x (1day/24hr) x (1yr/365d)]
x [(1hr/day) x (365days/yr)]
line 1: events per year per station (chance of being in the right place;
line 2: fraction of year represented by (death + 15 minutes) (chance of being there at the right time;
line 3:amount of time I am in subway, (opportunities I had to be in the right place at the right time).